Keith has at least two regular Telecasters with only 5 strings tuned to GDGBD, and likely more guitars than we can imagine.

As for G major chords, context is everything, dictating "voicing", fingering, and number of octaves. Remember that if it occurs in one place on the fingerboard it has to occur everywhere else. Highlight all the G, B, and D notes on a fingerboard map and G chords will pop up all over the place. As for which one to use, the ear will help.

Peace, Mooh.