From what I can tell, they are seen as a mouse. With the guncon2 driver, the gun acts like a mouse in windows, and when setting up the buttons, they registered as mouse buttons.
So, if you you mame.exe area51 -mouse
you should be set to be able to use these. You might need to recalibrate once in the game, but other than assigning buttons for coin, start, and button1, that's all I've done.