If you are a beginner, you may wish to simply play the G minor pentatonic scale in strict time as the chords play (as is suggested in the slow blues in G found in the jam-tracks section). Your ear will testify to the forgiving nature of this important standard progression. As you start to mimic and develop phrases you will see that ending a passage of notes with the root of the chord (play a G note against the G chord) will give the phrases a strong melodic sensibility.

As with most blues progressions, the chords are all played as dominants and will support both minor and major pentatonic scales. If you have practiced these scales, and have a smooth flow you might try addressing each chord with the scale of the same name (G minor or major pentatonic over the G7 chord, C minor or major pentatonic over the C7 chord).

If you are becoming an accelerated player you may utilize the modes of the major scales by playing up a 4th from the current chord. This means that all seven modes of C major can be played over G7, the modes of F major apply over C7, and the modes of G major apply over the D7. This is demonstrated in the single chord performance notes from this page's source and an introduction to the modes appears in the free lessons section.