I think people around here can't really help you because the code you use has never been converted to work with 3DGS afaik...
The old wiki unfortunately has some shaders that appear to just have been taken out of a program like ATI's Render Monkey without having ever been converted. Probably you could check out the new wiki for shaders that should work with 3DGS.
Other than that, I'd really like to help you, but I don't have any deeper knowledge about shaders... Sorry.