there haven't been any D2 or D2G updates since their releases that are to any serious significance. So it really cant be known yet. The devices are quite similar so the ROMs aren't that different. But they are different enough to specify them as different updates to different devices. So i would expect to see the D2G updates roll out about 2-3 weeks after the D2. I only assume the D2 gets it first because of how similar the ROMs are to the Droid X.
I second that: D2 first, followed by D2G. The hardware in the D2G requires a different ROM so it can deal with SIM cards and GSM calling--which the D2 never has to face. Rumors have the D2 getting 2.3 "soon", which HOPEFULLY means the D2G may get an update...by Christmas.