just to confirm. do you have a cd/dvd device in your pc ?
If you mean doesn't work the executable that make your gp3.exe patched (any errors ? mind you need admin privileges to run) , or otherwise it said all ok , but when you run the GpxPatch
it said to insert cd anyway ? in this case could be due to not installed any physical device into the pc, or more rarely a runtime protection antivirus of windows .
For the first problem you could sort with MagicDisc , it will install a virtual Cd device in your system , where you can also use to mount isos files or generate them. When you have the virtual unit repatch your unpatched gp3.exe and that should work.
For the problem of Antivirus runtime protection just add an exception on your GP3 folder to avoid problems.
Hope that helps. let us know which kind of problem you had.