Asklemmy

Sure. But when I say, "professional hit man" I don't mean a gangster, a mobster, or a spy. I'm referring to more of the professional hitman as seen in popular culture: https://tvtropes.org/pmwiki/pmwiki.php/Main/ProfessionalKiller

Think the trope in fiction. Someone wants someone killed. They find the shadiest person they can, like their tweaker cousin's dealer. They then ask this shady person if they can put them in touch with a contract killer. Through the grapevine, they meet with someone who is literally a professional killer-for-hire. The usual trope is some extremely well-put together gentleman; he probably wears a 3-piece suit and black leather gloves. He probably views killing as an art form. He takes professional pride in it. He's probably obsessed with expensive firearms and their various accessories, and he personally owns an arsenal big enough to take down the government of a modestly-sized city. Killing is his passion; he only charges at all because he has bills to pay like anyone else. The usual trope is to imagine someone as professional and presentable as the most formal lawyer or doctor you can imagine, except their business is killing.

While the real world version of a professional killer wouldn't be so extreme, the core should remain if they are to be a professional killer. They don't need to dress in a suit, have a private arsenal of rare expensive weapons, and speak in a British accent. But they should still meet the minimum definition of professional to count as a professional hitman. A "professional" is generally someone who offers a specific service to the public as their primary occupation. Lots of people know first aid. But only a doctor or a nurse makes medical care their actual profession. Most people can replace a light switch, but that doesn't make them a professional electrician. A professional usually sees some higher purpose or artfulness in their services and seeks to provide them to all that can afford them.

Sammy Gravano was a mobster, a terrible human being, and a ruthless killer. But he was a mobster first, and a killer second. He committed numerous other crimes on behalf of the mob, not just murder. And he didn't commit murders that weren't at the behest of the mob. You, as a random stranger, couldn't just knock on his door, hand him a bag full of cash, and get him to off someone for you. In fact, he would probably kill YOU just for trying.

Per OP's original question, Sammy Gravano does not count. There was no way for some random person back during his day to find him, hire him, and have him take someone out for them. He was a mobster, not a professional hitman-for-hire. And that is a crucial distinction.