You have a 4 wire hook up currently in place with a 50 Amp GFCI. You are not missing anything yet, but you do have an extra wire. The extra wire is the white neutural wire that is attatched to the "load screw" on the actual breaker itself.
Your previous set up has 4 wires connected to the GFCI.
1 wire is the curly pigtail wire that is white and permanently attached to the circuit breaker.
There are 3 "load screws" on your breaker and three wires attatched to them. 2 of the wires are next to each other and they are the 2 hot wires and carry the 110 volts on each wire providing 240 volts. They are usually black and red, but the electrician could have used both black.
The third wire attached to a "load screw" would be a white wire which is the neutural wire and IS NOT USED IN A THREE WIRE HOOK UP. It should be disconnected from the breaker and "capped off". The same should be done in the Hot Tub control box when you are hooking the wires up. The 2 hots go to the teminal load hook up on TB1.
The ground wire to the common buss bar, and the WHITE WIRE IS HOOKED UP TO NOTHING, "cap it off".
Thus, you will have 3 wires hooked up, 2 hots and a ground wire. You are done.
IF the white wire remains connected to the breaker "load screw" and/or the common buss bar in the Hot Tub load box, you WILL have nusiance tripping of the GFCI.