The place to start is with the doors to the cab itself. The gaps to check are all around the door, bottom edge to rocker, back edge to the body and around the upper perimeter of the door. Once the door is set then the fenders are next. There are shim points top and bottom by the door edge. It appears that it needs shim(s) removed to snug up the hood to fender gaps. I've seen shims at the fender to core support, but never from the factory.
I would measure the overall width of the old hood and compare that to the new hood just to make sure the new hood isn't narrower than the old GM hood. If the width checks out the gaps should be able to be snugged up.