Ok that's the situation we mentioned above. The upgrade that broke my gdm was the one including gtk+2.8.17 and the one before that (I applied them together). I see now that one of those updates included libgsf, the library mentioned by satinet. He says he gets libgsf related errors. I said my gnome was working, but I checked again and yes, nautilus gives a libgsf error here too. It's likely that libgsf is the cause. I've checked gtk+2 by downgrading, and that didn't fix it. That was the only thing (that I could see) among the libraries linked to gdm that got upraded in the batches i mentioned above. But it seems gsf is required for gnome in the background, so it could be the reason.