So, I'm on Mac OSX 10.8.5 and I would like to host a multiplayer Baldur's Gate 2 game. I've heard all my friends recommend GameRanger for this (http://www.gameranger.com/about/) and I've succesfully downloaded and installed the Mac version of it.
Unfortunately, when I try to host a game, GR does not recognise any of my installed GOG games (BG1, BG2, HoMM3 etc). These are supported on Mac according to the GameRanger website. The game appears greyed out on the list, and the same goes for manually attempting to add the game.

Are the Mac versions of GOG games not supported in multiplayer, specifically with GameRanger?

I'm not sure, I don't have a Mac, but did you try adding your games on the list of games to host? Sometimes you need to manually set up the path of the files for Gameranger because it can't find them.
EDIT: Oops, sorry, I didn't read your post correctly! Looks like you already tried that!