It is a colloquial term most often used in the west of Scotland, and Glasgow especially, to describe an act of boldness or daring (eg. “The way she spoke to Mrs Waters was gallus”).
As the example implies, someone who acts in a gallus manner is usually doing something that they shouldn’t be; formative usage of the word was more condemnatory in nature, refering to someone of dubious character who was fit to be hung from the gallows.
In contemporary terms, it can also refer to someone who does something in a stylish or exemplary manner; quite a turnaround for an adjective that would nowadays sooner see you wearing a medal, rather than a noose.
https://www.scotsman.com/heritage-and-retro/heritage/scottish-word-of-the-day-gallus-2463908
