Matthew. Yes, in the key of G 1.4.5 chords arw G C & D. In the key of C they are C, F & G.
Take the G major scale: G A B C D E F# G
You can see the first note is G the second is A etc. The 1 chord is G the 4 is C the 5 D. as a matter of interest the 6 chord is Em which is the relative minor in the Key of G. You will find that if there are other minor chords in a song they will usually be the relative minor chords of the 4 & 5 chords. So in the key of G the other minor chords that could crop up would be Am the relative minor in C and Bm the relative minor in D. I tend to use the 1,4,5. term because we all can,t sing in Gordon's key, I do The Auctioneer, without the capo, and use G, C, D & A ( 1,4,5 & 2)