Hi all, been wrestling with the idea of tone centers and keys . Do they always align? which do I tie my solos to?

Example: Sweet Home Alabama

Chords: D,C,G G = tonic; C=subdominant 4th; G=Dominant 5th; Indicates Key of G

Use G scales..

Playing the song:

Starts on D Chord and seems to resolve there

so indicates D scales should be used

So, how do I think about this? Key of G with a differnt tonal center being D and use D scales? Am I incorrectly assuming a functional harmony progression for this song and it is really a modal progression?

i would be thankful if some one could get me straight on this ;-)

B