G lydian scale instead of G major scale over a Gmaj7 chord?

That sharpened 4th (C#) makes A LOT of difference. Can someone explain to me why that is. Whenever I play g major scale over the gmaj7 chord I always avoid that c, unless I sharpen it. Odd since the 4th step usually sounds nice over the I chord. But when you add the F# (7th) in the chord then it sounds TERRIBLE. F# and C don't go along well in this context it seems even though it's IN key. F# has c as it's b5 interval, you'd think they'd would be pleasing to the ear and not dissonant.