Because of the Mac release of Steam (if you haven’t already heard from everybody and their mother), Valve are giving away portal - for free!
Go here, download, install etc and voila, free portal .
I cannot guarantee that this will work as a source game for Garrysmod, as it’s technically free and most likely doesn’t provide the SDK with it.
This isn’t Portal first slice, it’s the actual game of Portal which you’re allowed to keep even after the free period I believe. So have fun.