Are there any mac os x games on gog.com? I've tried "search" and didn't find anything.

The games that run through dosbox or scummvm can run on Mac. Other than those, nope (if we exclude virtualization and stuff).

A more accurate description would be to say that the games which use DOSBox or ScummVM can be made to run under OS X with the help of the OS X versions of those two programs. The game installers themselves cannot be run natively under OS X however, as they are Windows applications. You will therefore need some way to get at the game files inside the installer.

Be advised that GOG does not support OS X, so any problems you may encounter along the way are your own.

There are no native OS X games on GOG.

For non-DosBox, non-ScummVM games you will need a program like Wineskin or CrossOver games. These also allow you to run the installers on your computer without a Windows partition or virtual emulation environment. From the FAQ thread here are some helpful links:

Are GOG's games Mac/Linux/other OS compatible?
GOG games are officially compatible Windows XP and Vista only (Windows 7 compatibility is a work in progress and differs from game to game). Some of the games available from GOG are known to work on *nix systems using Wine and the native versions of DOSBox and ScummVM. Details on Linux compatibility can be found in Lexx's GOG Games that are working fine with Linux/ Wine thread and in paldepind's Great GOG games that work on Linux GOGMix. Details on Mac compatibility can be found in GOGMixes by NoanRand and rampancy and on PaultheTall's website. General information on installing and running GOG games on Macs can be found in rampancy's GOG Games on OS X: A How-To thread. Please note that GOG does not support using these games on any OS other than Windows.