If I remember correctly, you may have to select the correct DLC/expansion when initially setting up the game on multiplayerrobot.com. If you don't select the appropriate DLC or just select the vanilla version of the game, I don't think GMR will let you select DLC/expansion civs. If you are at the joining/selecting a civ phase of the setup process and you don't see it set up correctly, you might have to start over the process... can't be certain though lol. Been a while since I set up my own game.

I’ve gotten the same crash and error message before on a GMR game. Not sure what exactly caused it, but reverting the turn and having the previous player redo his turn fixed it. My guess is that the save file became “bloated” due to something the previous player did (or didn’t do). You can do a google search about civ 5 GMR save file bloating and learn about it. There are ways to unbloat a save file if the previous player doesn’t want to redo (or if it comes directly from the live hotseat game). Hope this helps!

I remember reading somewhere that some mod files are more complex than others, and that only the simpler mod files work with GMR.

Looking for 5 substitutes for a 12 player, huge size map game! Available civs are as follows:

Morocco
Poland
Babylon
Mongolia
India

Not sure if it's public yet, but follow this link and join/comment about joining:
http://multiplayerrobot.com/Game#22767