Thomas Berglund is a musician that has played in many different styles through the years and he has his heart in music with improvisation.
The soloing chords are:
You can play G Ionian scale (Root - 2nd - major 3rd - perfect 4th - perfect 5th - major 6th - major 7th) and G major pentatonic scale (Root - 2nd - major 3rd - perfect 5th - major 6th) on the G chord.
//: G / C7 / D7 / G ://
On the C7 chord you can soloing on these tones (Root - 2nd - major 3rd - perfect 5th - major 6th - minor 7th). I call that scale "mixolydian no 4th" but if you will call it with another name it's ok. If anyone has a better name please tell me. You can also soloing on the G minor blues scale though C7 is a kind of a bluesy chord in the context. That depends on the minor 7th tone in the C7 chord that's the minor 3rd tone in G and since the tune is in G major key the minor 3rd gives it the bluesy touch. The blues scale in G minor have the tones from G: Root - minor 3rd - perfect 4th - perfect 5th - minor 7th.
On the D7 chord you can play D Mixolydian scale (Root - 2nd - major 3rd - perfect 4th - perfect 5th - major 6th - minor 7th) and the major pentatonic scale (Root - 2nd - major 3rd - perfect 5th - major 6th). The root is the tonic tone in each chord!
In my solo I'm playing some chromatic tones and I'll end the solo with the minor 7th tone on the G chord and that makes it a dominant chord that leads good to the next chord that's C7. But I have the scales I show in mind when I'm soloing.