Well, the mode most often associated with Garcia is Mixolydian. Jerry does also use the straight Major ( and Major pentatonics ) alot.
For example, over the B-A progression in "Fire On the Mountain", the chords should be treated as a V-IV in the key of E Major. Therefore, use the B mixolydian over the B part, and I like to use either A Major Pentatonic or the more correct ( and colorful ) A Lydian mode over the A.
There are also some songs where he uses the Dorian mode over minor chords. To really help nail Jerry's sound, though, you want to concentrate on playing chord tones more than anything, so learn( if you don't already know them ) some arpeggios on different string sets.

