Why can't I play this game with a joy pad?
Why doesn't the editor I downloaded with the game work?
Why can't I play it in full screen?
Why is this such a shitty download that takes a lot of work to run with almost no help from anyone?
First off, be more respectful. No one likes rude folks.
Secondly, the Duke3D.GRP file is in the game's install directory, which you can find after installing it by going to C:\Program Files (x86)\GOG.com\Duke Nukem 3D on the GOG.com version.
As to controller mapping, that's an issue with the fact that the game was made well before the advent of USB, so USB controllers aren't going to work without wrangling around in DOSBox a bit. Of course, you can slip the GRP file into a source port and see if you get better luck there.