I know you realise this but try to avoid using JoyToKey. I have found devices that do not work with Mamewah, but I reckon you should be able to get your GPWiz's to work. First just as a check, have a look in your mamewah.log file, and check that your joystick devices are being enumerated...and also take note of the joystick device numbers (may have to enable joystick first if you haven't already, see below).
In your ctrlr file (name & location varies with Mamewah version), check you have 'joystick 1' set under the ### SETTINGS ###, and also 'joy1_x_analog 0' and 'joy1_y_analog 0', assuming 'joy1' is the one you want to use. Do not enable 'analog' since the analog support can be a little unpredictable in Mamewah at the moment.
Finally just make sure your joys are mapped to some actions, eg:
UP_1_GAME "DIK_UP