i'm not good with harmony my-self, but you dont replace the G, you put a harmony beside it! eg; if you sing (or play) G and D at the same time you find it's a natural harmony, which is why we get chords.
eg; chord of G is: G-B-D-G-B and high G. so basically the only notes you play are G-B and D in different octaves.
you can expand on this by playing F in place of the high G, which obviously gives you G7. which sounds nice when you sing those notes. other harmonic notes for G would be: E (for 6th) F# ( for major 7th) A (for 9th) these will come with practice.
hope this helps.
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