Hello! I'm having the exact problem as OP was but unlike him, I can't find a fix. I have the Driving Force GT pedals too and when I try to assign the rudder axis to my pedals only one axis at a time is allowed or in other words, if both are assigned they go haywire when I'm in-game.
If I go to the other option of "left rudder" and "right rudder" and try to individually assign the pedals there, the pressing of my pedals doesn't register and nothing happens on the keybind.
I've tried what Razor5-1 said to do in the post above mine, but so far I can only get one axis at a time to work and its not easy to use as you have to keep one pedal half way depressed to have a neutral rudder.
Please offer any help!
Thank you!