I'm not sure what GP is but I found your forum while checking out my counter thing and finding our where people were linking to me from.
I see some are having problems.
Here's some tips that might help.
Make sure the wad you are using is for the game you are playing.
If your GP thingy is playing regular doom then don't bother trying to load a wad made for Doom2 - it won't work.
If the wad you're trying to load requires a port of Doom - like Zdoom - don't bother, unless you can load zdoom as well.
I wouldn't think that new sprites, textures, etc... would be a problem - especially if you can play simpsons doom.
In fact, it sounds like maybe you guys are using the original Doom and Not Doom2.
If that's the case you will probably have problems with alot of total conversions but you shouldn't have any problem with most regular doom wads.
Try this part of my site:
http://www.doomwadstation.com/doom_reviews
Please, let me know if I can be of any assistance. I'm not familiar with your system but I know doom.