Most traditional rock songs are essentially composed of the 1,4,5 chords. Think Wild Thing, or La Bamba, or thousands of other famous songs that are basically 1,4,5 chords repeated. This is because rock progressions are generally derived from blues music, which also predominantly features 1,4,5 progressions.
Now bluegrass and country music are also derived from the blues and predominantly feature 1,4,5 progressions too. So typically a blues song or a bluegrass song in G will be G,C,D. The difference might be that for blues, you will create a melody over those chords using the blues scale (G,Bb,C,D,F), while for bluegrass you will use the G major scale (G,A,B,C,D,E,F#).
Compare the differences in the two different scales; especially the F vs the F#. One of the main notes that make the blues scale in G sound so unique is the F, or flat 7 note. It’s so much a part of the blues sound that typically blues songs will feature dominant 7th chords when playing the 1,4,5 chords. Dominant 7th chords are a major chord with an added flat 7th note. G7 is made up of G,B,D,F notes.
Where this gets interesting is when you encounter a song that’s halfway between blues and bluegrass; what i call swamp music. Can’t You See, and Sweet Home Alabama are examples of this. These songs feature a chord progression of D,C,G. Looking at those chords you might think, that it would be a song in G blues or G major since it uses G,C,D or 1,4,5 in the key of G. However, as you try to play the right notes for these songs you’ll will realize that they are actually in the D blues scale.
Blues and bluegrass songs in the key of D are supposed to feature D,G,A chords, but these swampy songs are D, G, C. I think of the C here as the flat 7 of the key of D. There are likely songs whose progression goes further to add an A or A7 chord at some point, but many don’t. I think of it as country blues, or swamp blues.
Interestingly, the name swamp music likely comes from the nickname of the group of rural Alabama studio musicians who changed the sound of R&B and soul music of the 1960s and recorded with most of the great soul artists the the day in the famous studio at the edge of an Alabama swamp called Muscle Shoals. They were called “the swampers”.
