Well, all of those chords in the progression are in the key of G major. So... you can use a G major scale OR the relative minor, which would be E. E minor has the same notes in the scale as G major, so it is considered the relative minor of G major. There are certaintly other scales or modes to use, but in this case where the chords are all strictly major, it would be best to just stick with G major or E minor (pentatonic, if you wish)