Gxx is the note delay effect. It sets the delay before the current row is triggered (anything sound that was playing until the row it's used on will keep on playing until the delay is over).
xx = amount of time to wait (delay in NES audio engine updates). When xx > 150 (or 125 if PAL mode is used) × 6 ÷ Speed, the current row will be delayed no further than the next row.
C-4 00 F G03 --- -- - --- C-4 00 - G06 G-4 00 - ---
C-4 and G-4 are notes, 00 is instrument number, F is volume. Tempo set to 150, speed set to 6, NTSC mode used. The first C-4 is delayed by 3 frames while the second C-4 is not audible because it is forced to be played in row 03 but there is already a note.