Egads. First off, this is definitely a NEWBIE question, and a FREQUENTLY asked one as well. Using the Search button, I guess, was too hard.
Secondly, even if you felt it wasn't a "newbie" question, it's STILL a DVD question, and there's yet another forum for it.
As for GI files, those are Global Images, used by Prassi PrimoDVD and Veritas RecordNow/RecordMax (which is just a newer version of Prassi's PrimoDVD). Sadly, ISOBuster cannot handle multi-part gi files, so if you have files with .gi(x) ISOBuster will NOT help you.
Likely you downloaded Space Ace or Dragon's Lair DVDs. These are DVD movies, not Xbox games (Though there *is* a Dragon's Lair game for the Xbox). You cannot "play" these from the hard drive, so if you do not have a DVD burner, FORGET it; you will be wasting your time.