ok, I may be a bit confused.
Let me distinguish 2 problems:
1. windows virtual cd drive (you need anyway to play with Gp3, also if you have patched GP3noCD. Typically windows 8 or 10 can mount a disc dvd by an iso, any iso, it will create he virtual drive and mount in it. Windows 7 you can install MagicDisc for that (the app will create a virtual drive letter where you can mount the iso image). Anyway, for this problem you will need the system have a cd/dvd drive letter.
2. For gp3 nocd (utility to play in gp3 without gp3 cd needed, but you need to have a cd drive in the system) , install gp3 (Full mode), by the link above get the gp3 113.zip (unzip/overwrite the file gp3 version 1.13 over the 1.00 in the folder of the game). then you do not need the cd/iso file of gp3 inserted/mounted to play. It should start without.