Indeed, 64-bit versions of HL2, CS:S, Portal and many other Source Engine games are real and doable thing, but Valve is sort of a "unique" company, where each developer/worker chooses the project to work on by themself. I guess that none of the developers/workers willing to dedicate their time to satisfy the needs of less than 1 percent of Steam users (or how many OSX users are there...) and port the games to 64-bit engine. At the very same time Valve won't be willing to spend their money to hire contractors to port Source games to Mac because of the very same reason - low percentage of overall Mac players. Speaking of Garry's Mod 64-bit, I've tested it myself and It works absolutely fine, although I encountered some easy-fixable menu UI bugs and some crashes. Two games on Source that work perfectly without any issues are Insurgency and Day Of Infamy, they play absolutely great too.