I have Garry's Mod for Macintosh. I can download maps with ease, but I'm unsure how to download general mods and SWEPs.
Here you have disscusion on Steam Support about it.
Go to ~/Library/Application Support/Steam/SteamApps/[your username]/garrysmod/garrysmod/addons
Place the folder containing the info.txt file in that addons folder.
( ~ means your user directory, your "home" directory)
Hope it helps.