When you see G7, that means G dominant 7, not GMaj7. The chord would be built:
F (Natural)
D (you can throw in Db's here and it'll sound nice and bluesy)
B (Bb will make a more bluesy sound, but it's best used when traveling between notes, like going in a melody:D B Bb A G)
G
You can turn it into a scale and think about writing melodies using only these notes:
G A (Bb) B C (Db) D E F (F# - sparingly, usually only if you're heading straight to the G right above it) and G at the top.
And when it goes to the C13, you would use the same scale, just focus more on the notes of the chord. In other words, you could still use ANY of those notes in the scale, just try and use C E G Bb etc more, to establish a chord.
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