A g chord (as any other standard maj or min chord) is comprised of only 3 notes. You can substitute these notes any way you wish.
And yeah, I learned a g chord almost the first time i picked up the guitar. First chord ever. Not bragging, but it just seems like one of the easiest after E minor. F chords took me soooo long to learn though. OP, just keep switching from all the chords you know to G very slowly. If you have the patience to do it for a half hour, you will get it.