Can i use G major 6th in the of C?

i know the chords in the key of C major are Em, F, G, Am, Bm, C, Dm. And i know there is some reason certain chords can be 'substituted'. But since im self teaching myself and enjoy figuring things out myself before confirming it (i find it cements it in my brain better), my learning is slightly fragmented. so i was just wondering if G major 6th can be subbed for G major or Em in a chord progression.