A bunch of stages for starters and as a result a different circuit.
The 69 is more about guitar tone than amp sim. The G.B.D. simulates the tube powerhouse amp as opposed to the the pure guitar tone of the Stones who used prototype SVTs on the Ya-Ya’s tour. Again the 69 breaks up like you would expect a guitar pedal to do and is all about that cranked air pushing tone without needing the volume. As a result you also have a lot of useful in between tones that the G.B.D. does not yield. Coincidentally this why the G.B.D. is a much better circuit for Bass guitar.
Two different monsters really 😉 I hope that helps.