Nothing... OMIT IT. There's NO neutral that runs to the spa.

Ok, from the info provided I have several solutions. If you have a 4 wire connection from your GFI running to your spa, you need to look in your GFI box. Unhook the white wire on the voltage side of the breaker. Make sure you have a neutral white wire hooked up on the in side though. If you do not have a GFI box, then your options are to go into the main breaker box and adjust it for 3 wire, I highly recommend installing a GFI box within 5 feet of your tub, any other questions. Please don't hesitate to ask

If it's 4 wire then it probably has some 110v components(like an ozonater or control panel) but the ground wire is put along with the neutral and performs the same purpose.

A 4 wire connection should be run like this IF I am not mistaken. There should be 3 connections on your spa heater when you take the cover off. They are marked NEUTRAL(Return to the same ground bar as the bare ground) HOT(From one post from the breaker 110) HOT(From the other post of the breaker 110) Then there will be a ground bar on the outside of the connection box you run the bare (4th) wire to the ground bar in your breaker box.