This a very late reaction (I'm quite new in here) - hope it might be useful anyway:
GP750 and GP790 behaves very different in Limit setup. Both in terms of programming and in terms of the movements of the tail servo in the setup situation ("normal" for GP750 and "sticky" for GP790).
GP750
Via rudder stick (on TX) first go left to max. desired position. Then do similar to the right. Return to the center position, and press the SET button. Done.
Note: The servo movement is "normal", and not "sticky" (opposite to the GP790).
GP790
Via rudder stick (on TX) first go left to max. desired position. The servo movement is "sticky" (opposite to the GP750), and can be adjusted slightly back and forth around the outer position. When this looks sensible, you stop the input from the transmitter, and after a couple of seconds the lights flashes red. Then do the same to the right, and when the lights flashes red again (in this right position), Limit is set for both left and right.