Jim, There's a really good book you should invest in. It's 'The Banjo Case Chord Book' by Larry Sandberg published by Amsc Publications. I've found it real helpful.

I assume that by GModal you're talking about tuning gDGCD. I play clawhammer, so I don't think in terms of chords that much, but I've got my banjo on my knee and I'll give it a go... For a G chord, try playing open or fingering the high D string at the 4th fret. G - 00004 C - 00545 D7 - 00200 F - 03203 A - 02226 (barre this one at the 2nd fret)

Hope this helps out.

- Sean