I'm trying to create something with 3D, never used this feature before. I'm trying to change the grey surface to a gradient. How can it be done?
This is what I'm looking at
Create a Symbol for your gradient (just a rectangle with the gradient applied will do fine). You can then apply your symbol to one of the object surfaces using Map Art... in the 3D Extrude & Bevel Options.
Use Scale to Fit to stretch the gradient to the correct size of the suface. You'll have to do this for every surface on your shape.