Very few are genuine Mac versions, most indies probably are, but since Apple switched to Intel in 2007 (unless I remember wrongly) and the PowerPC emulator (Rosetta) was axed with 10.6, only Mac games released in, say 2008 and later, stand any chance of running on current machines. Anything else are either DOS version running in DosBox/Boxer or Windows versions running under Wine, or games that run under ScummVM.
Games from before OS X definitely won't run without bundling an emulator, since Classic was never ported to Intel (though it was bundled up until OS X 10.5 for PowerPC). And the easiest solution is, frankly, to run the DOS or Windows version through their respective solution.
Games for System 6 and 7.0 (I doubt 7.1 or higher) could possibly be emulated though Executor, but that'd require quite a bit of work on GOG's part (the Executor source would likely have to be updated heavily) so I doubt we'll see games in a similar situation as 3 in Three here any time soon (that is, games that were only released for Mac and do run well under Executor - but my tries in making a GOG-like setup fail to get sound working even though it does work if I launch the emulator first and run the game through it).
Post edited February 04, 2013 by Miaghstir