There may be an adjustment on your GPWiz49 to do this (I'm not familiar with the product), but if you use wires to pull all of the input lines (everything but power and ground - so pins 2-9) to ground, that is the signal 49 way joysticks use for "center". If you want to be completely proper about it, chuck a 100 ohm resistor in there between ground and the bundle of all the inputs, but nobody will tell if you don't.