G C D. That’s the 1 4 5 in the key of G
G Em C D that’s the 1 6 4 5 in G
If you know the F chord
C F G 1 4 5 in the key of C
C Am F G that’s the 1 6 4 5 in C
These are common Chord Progression in thousands of song using the C chord.
You can practice using any one these chords with C or practice the chord progression
and see what song you come up with.