Yep, fenders are interchangeable, as is everything else from the firewall back - both inside and out, with the exception of the dash on certain years. As mentioned before, the hood, grille surround, and grille are not interchangeable. Headlights interchange, but parking lights/turn signals are just different enough that you'll need to change both if you decide to go that route - they'll bolt up, but they look different enough that you'll notice it from a mile away if you have one of each. The gravel pan that goes between the bumper and the front sheet metal is a bit different too, but it will swap from Chevy to GMC and vice versa with no problem.
Lots of people who can't find GMC front sheet metal have gone with Chevy front ends as the GMC grille surrounds, hoods, and grilles are getting hard to find - comparatively speaking, that is. If you decide that you have to go with a Chevy grille and hood, be ready to stop calling it a GMC to everyone except the DMV - unless, of course, you like to explain to people why it's a GMC with a Chevy front end…