Griff discusses this in the section "Borrowed Arpeggios" back on page 20 in the manual and in the Section 4 video.
G Maj7 is the IV chord in the Key in which A7 is the V chord, i.e.
D Major.
The A7 chord is the V chord in the
Key of D.
The IV chord in the
Key of D is G Maj7.
You are playing the G Maj7 arpeggio over the A7.
You "borrowed" it from the Key of D; the same Key A7 is actually in.
Also:
A7 =
A C# E G (1 3 5 b7)
G Maj7 =
G B D F# (in A, these are the b7, 9, 11 & 13)
By playing the G Maj 7 arpeggio over the A7 chord you are adding the 9, 11 & 13 sounds to the A7 chord.
Combined, they are all notes of the A Mixolydian scale : A
B C#
D E
F# G
You're just "seeing" and playing notes of the A Mixolydian scale in a different way, a G Maj7 arpeggio over an A7 chord.
If you stack them all in 3rds you have an A13 chord: A C# E
G B D F#