Like everyone said, it's best off you virtually run Windows 10 which is quite easy to set up.
Garry's Mod and gaming in general is terrible for macOS, you do get games that aren't even supported for macOS so this will be a great help for other games you may want to play.

Like everyone said, it's best off you virtually run Windows 10 which is quite easy to set up. Garry's Mod and gaming in general is terrible for macOS, you do get games that aren't even supported for macOS so this will be a great help for other games you may want to play.