Are you wanted to know what chord(s) that you can play along with a solo in the key of G major?? If this is what you mean, you can use any of the chords that are in the key of G major. (i.e. G major, A minor, B minor, and so on.) I may have misinterpeted what you meant. If your soloing in any scale, you can use any of the chords that are that scale. It's called harmonic generalization, when you use one scale over a group of chords.

It works the other way around too, you can play any note (even just one note) in the G major scale over a chord progression in the key of G major. As for which one is "the rock tone you're looking for", you would have to be the judge of that. Just try all the tones til you find the one your looking for. I couldn't give you a straight answer cause both the chord progression and the note you use would be a factor in it's harmonic sound.

Modes on the other hand tend to work (or understood) better when matched with the chord you are using it to harmonize over. It's more complicated to think of them as general harmonization because if you play a C Lydian scale over a G major chord progression. Your really only playing the G major scale over the G major chord progression. Alot of people wonder what the use of modes are because of this. If you would play the G lydian mode over the G major chord progression, you would end up with alot of clashing of notes. Take the chord progression you wrote G, D, C. G lydian carries the note C# so everytime you played a C# over either C or D, it would sound wrong.

Now if you use the G lydian scale over the G major chord, you would end up with a major scale with a raised 4th. That's the tonality of the lydian mode. All the other notes in the mode would sound like a major scale, except that 4th which is very colorful. The regular 4th is rather dull sounding (I think), and it clashes with the 3rd in a major chord.

There's two ways to look at it. First is general harmonization which is using the root scale over a chord progression in the same root key(using the G major scale over a G major chord progression). Then there's chord harmonization where to use different scales or modes over each chord depending on the harmonic quality you want (i.e. using the G lydian mode over the G major chord).

I know I kinda dragged on but that should answer what I think your trying to ask, and probably any questions after.