Does Garry's Mod run on Mac?

Yes — Garry's Mod has a native Mac version.

Start there, not with us: Steam lists a macOS build of Garry's Mod and your Steam purchase includes it, so install that first. We would rather tell you that than sell you a compatibility layer you have no use for. If the native build turns out to be an old one that no longer runs on modern macOS, Pixel Port runs the Windows version instead.