Here is a free, printable diagram showing the Diatonic Chords in the key of G Major.
One thing you may notice is that the V chord is minor. You can keep it as a minor chord and it will sound good, but typically the V chord wants to be a dominant chord. It is common practice to change the V chord in this case to a D7. Try it out and see what you think.G Minor Diatonic Chord Scale