There are some Mac mods for Fallout 2, but that's probably not what you're looking for. http://www.nma-fallout.com/downloads.php?do=cat&id=51

But here someone is working on Fallout 1 FIXT megamod for Mac - http://www.nma-fallout.com/showthread.php?50313-Fallout-1-FIXT-megamod-for-Mac-OSX/page2

GOG's Mac version of Fallout is actually the Windows version running in Wineskin (an app to run Windows programs on Mac). Hence you can't use any mods build for the native Mac version but you can use the Windows mods. You have to open the package and browse the package content to see the game's original folder structure. Then you can copy the files of any mod inside. Search the forum or Google on how to install them using Fallout within a Wine wrapper (Wineskin). It's possible and there are some instructions out there.

For Fallout 1, basically all you need is the FIXT mod (with or without additional content).

GOG's Mac version of Fallout 2 is also the Windows version wrapped with Wineskin. You can open the package and copy mod files there as well. There are also some threads about how to do this here in the forum. Basically the only mod you need for Fallout 2 is Killap's Fallout 2 Restoration Project (There are also 2 versions, Killap's Fallout 2 patch with fixes only and the complete Restoration Project with additional content).
Here are tech infos on how to install it with GOG's Mac version:
http://falloutmods.wikia.com/wiki/F2RP_Technical_Info