I'm not an expert. But I believe it is something to do with information redundancy.

If you mishear a word but surrounding words must match gender and number, you may reconstruct the misheard word.

As a native spanish speaker, I don't think of the actual sexuality of objects, it's just a characteristic of the word that should match other words in the sentence. For example the word screen (pantalla) is femenine, and the word monitor (monitor) is masculine. So when I see my monitor I don't think of an actual female or male object. But the nouns should match adjectives gender, so if someone says "broken monitor" (monitor roto) or "broken screen" (pantalla rota) I have this kind of redundancy if I misheard a word.

But I'm not an expert of linguistics. Don't quote me.

Speaking as a gendered language user (Italian) it is sometimes weird.

For example, car is feminine but our name for an off-road vehicle is masculine, as is the word for truck. Since you have to apply the gender of the noun to verbs, articles and adjectives, which one do you use when talking about your SUV? Feminine because it's a car or masculine because it's an offroader?

For borrowed words there's usually a consensus on gender that forms over time. Sometimes a borrowed word inherits its gender from the translation of that word that fell out of use. One example of this could be the word computer. An equivalent term exists in Italian (calcolatore) which fell out of use but gave it a definite gender, masculine.

What exactly does gender achieve in a language? Is English missing out on any nuance?

Sort of. Grammatical gender and the interplay with grammatical case (the "role" of a noun in a sentence) allows some extra meaning to be packed in. For example, German has 3 genders and 4 cases leading to 12 different contexts for nouns to be in. Many of those have their own conjugation patterns, and separate words for the articles "a/the".
That can, theoretically, allow meaning of the type "whose what did what to whom" to be obvious or pieced together in a sentence, whereas translating it into English you might need to spell it out, lose it, or rely on context.

In practice, a lot of that sort of information is often redundant or clear from context anyway, and only matters if you're being clever or succinct. My German is shit, so I will not try to provide examples.

It's also worth pointing out that it's a naturally occurring feature, likely arisen by accident.

Is it literally thinking about nouns as male or female, or is it just a weird name for the concept?

It is mostly just a weird name. Some of it makes sense along (social) gender lines, much of it makes no sense at all. This thread is full of good examples of counterintuitive noun genders in all kinds of languages.

Who decides gender when a new noun is made? What about borrowed words from other languages?

The speakers of the language, collectively, usually with some disagreement, trial and error. Borrowing depends: a gendered noun borrowed into a non-gendered language would just slip in there. In the reverse case, people would just arrive at some gender for it arbitrarily or based on similar words, what gender any "parts" of the term might be if translated, or whatever other method. There's no correct answer.

Do you sound stupid if you speak French without using it, or are you just a language hipster?

Quite likely. There's no "without it" in gendered languages, it is a more or less fundamental part of the noun and the language, like how certain nouns and verbs are just different in English. Dropping random grammar and syntax from English would just be "doing it wrong", ranging from cute foreign accent quirks to Ralph Wiggum's cave-dwelling ancestor.

Of course, fucking up is unavoidable when learning languages, and most people will give you a lot of leeway due to being foreign. Maybe not everywhere in France, though...

Gender from french genre, latin genus, means category and that's all it is, a category system, with confusing category names and no real rules for which word belongs to which category. There's nothing masculine, feminine or neuter about words, nothing "sexual" or whatever, otherwise every person would be a woman because the word for person (from latin persona) is feminine in a lot of european languages, or French and German people would have to think really different about stuff like tables because in French it's "feminine" and in German it's "masculine". Btw, looking at English adjectives with French origin they almost always are the feminine version, like feminine or masculine. Some people think there is a hidden sexual meaning though and they come up with lots of different systems for gender neutral language, stuff like latinx.

Russian speakers might say the same thing about things that exist in English but not Russian like articles (the words "a"/"an" and "the"), Afrikaans speakers may say the same thing about verb conjugation at all, Chinese speakers may say the same thing about tense, Japanese speakers may say the same thing about having a separate present & future tense. There is a good explanation here or two already, but language features that seem "useless" or "complex" to us are important in other languages and are there for a purpose. Every language has features that would make others question it.

English weirdly use feminine for ships, so think of it like that. But no it doesn't achieve much.

I don't think it change the way we think about objects much, but probably unconsciously yes. For example, France itself is feminine and seeing some caricature personifying as a dude always feels weird.

Usage dictates the gender. And some recent words are more or less controversial: gameboy, wifi, COVID, Nutella...

When I think about the gender of a word I will usually derive it from a broader category. But that's not always obvious, for example Gameboy is a game console (feminine) but the words game and boy are masculine. COVID is a disease (feminine) but also a virus (masculine). And in the meme a washing machine is a machine (feminine).

You can't not use gender since french doesn't have neutral pronouns. But I don't think it's frowned upon for a non native speaker to make this kind of mistakes.

You sound odd, like a child or someone not fluent if you don't use our misuse the genders of words.

That being said, as native Spanish that lived in the UK for a while, I noticed that genders and verb forms are useful for providing more context when talking.

Cannot think of specific examples now, but in general in a phrase if you don't hear a word or don't know the meaning, it is easier to guess it because the rest of the phrase is constructed around the gender and more complex verbal forms.

on borrowing we can look at nouns borrowed into Spanish. They take the word change any sounds in native language to match Spanish sounds. Then they just slap on a gender ending. Yes it just what ever catches on. Which means we could have lived in world with potata.

in greek (also gender for every noun, chair is female, dog is male and washing machine is “neutral”) it’s weird when someone uses the wrong gender. mostly non-natives mix them up

borrowed words are mostly neutral like μπουζί (spark plug) or ντουζ (shower)

as to what’s english missing I’d say complexity. Learning english and just being able to throw “the” behind any inanimate object is amazing. Also learning the genders that differ from your own language (learning french for example where a cat I think is male but in greek it’s female) is even more difficult

As a native speaker of a language with grammatical gender (Croatian; I've also learned Russian and a bit of German)...

What exactly does gender achieve in a language?

In Slavic languages it serves as an additional syntactic "connector" between words. Masculine nouns are accompanied by masculine forms of adjectives, feminine by feminine, etc. (Other than adjectives, this also applies to pronouns, some numbers and verbs.) This isn't necessary for successful communication, but it can somewhat reduce ambiguity and, along with other trickier parts of grammar such as cases, allows for quite a bit of freedom in how a sentence can be organised. English can be limited in that regard, with its stricter rules on word order, although its lack of grammatical gender is not the most significant factor.

Is it literally thinking about nouns as male or female, or is it just a weird name for the concept?

It's more of a name, true. There are prototypical words and situations where grammatical gender really is the same as biological sex (e.g. when referring to specific real people - just as you'd call a woman 'she' in English, so do you have to use feminine adjectives when referring to her), and that relationship is, for the sake of simplicity, projected onto the entirety of nouns in the language.

Who decides gender when a new noun is made?

In Slavic languages, it's really simple because the noun endings usually correspond to gender. There are exceptions and, so to say, "subsystems" within the general system, and there can be changes in how that system works, but the point is that it's based on a set of rules that speakers do know intuitively.

German doesn't have such a clear system of genders that is visible within each word (the endings usually don't tell you anything useful; if the noun ends on 'e' it's relatively likely it's feminine, but that's about it, as far as I know). Yet, interestingly enough, there was an experiment where native Germans were provided with made-up words, and were asked to determine their gender. The majority of people agreed on their choices. So, clearly German does have some rules and procedures to determine gender, even if they're opaque.

What about borrowed words from other languages?

Same as above. I can provide some illustrative examples if you want?

Do you sound stupid if you speak French without using it

I tried to imagine some sentences of that sort in Croatian, with incorrect genders, and it doesn't sound outright stupid, just odd. Some situations allow for some leeway in choice of gender too, and natives can make mistakes if they don't think too clearly which word they intend to use, and none of that is especially bothersome to a native's ear.