A little theory to explain why the notes work with the chords.
The first note is the 7th of the G7 chord, and the second one is the root. The third note is a b3 for the G7 chord, and if you slide or bend into the note 1/2 step up (one fret), you play a famous "blues note" along the way.
At bar 6, we land on the C7 chord with a G note from the same phrase, which is the 5th of that chord.
In bar 10 we play a phrase something like this over the D7, which has a C note, the 7th of the chord, A# a #5 note which is a common "jazzy" sounding altered note played agains dom7 chords. We land on the G which is the 4th note in the mixolydian scale for D7, but it's really there to lead us into the C7, again G being the 5th of that chord.
In bar 11 we play something like this:
This is a repetative phrase over the C7 chord, with the 4th, 5th and 7th notes from C mixolydian.
Please check out my "Bone Head Blues" lesson #'s 8030 and 8402 for more beginner blues phrasing ideas. I have also begun a series about double-stops which may be used for blues.
Peace Hope Love and Joy