Watch the G9 Chord online guitar lesson by Marcy Marxer from Swingin' Rhythm Handbook
The G9 chord is a G7 chord with a 9 note added. Any chord with a number larger than 7 is a 7th chord with that note added. What about 11 and 13? Yup, 7th chords with the 11 or 13 notes added.
To find those notes start on the 1 note of the scale and count up to 8 for the octave, then keep on going.