i think you may be confusing scales and keys. If you start on the G chord, you can play any notes of the major scale in the key of G. As your chords change, your lead should try to target the root note of the chord....but you are still in the key of G. You can mix up the lead, but try to land on the roots of the chords at each change and it will sound like a natural flow. Let your ears guide you, if it sounds off, you're probably not hitting the roots at the right times.

Im sure there must be a lesson in here somewhere on "Chord Tone Soloing."