Those are following ,more or less, one of the most common chord proggresions. The I,IV,V chords. In this case there is also a vi chord on the end.
G=I, D=V,C=IV and Em=vi which is the relative minor of G.
C=I,G=V,F=IV and Am=vi which is the relative minor of C.
You would find these on a keyboard by counting up the white keys (tones)from G and the black keys(semi-tones) in the case of the Key of G progresion. The key of C of course is just the white keys.( no semi-tones.)starting with the c note.
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