ive never seen or heard of a gasket for a 7.5 or 8.8 rear. i worked at a ford dealer for over a year and we always used RTV.
I've put gaskets on both. They exist. I even found a nice reusable one for the 8.8 (but it is rather pricey).
Black RTV is a LOT worse on 8.8s, where you'll crack the plastic cover trying to get that SOB off. The 7.5s will bend up, but you can flatten them. Yes, that's experience.
As for using it at the dealer, they do go on that way, and they don't leak. But YOU don't have to take it off next time.
If you MUST use RTV, use it (very) sparingly. A big fat bead might not BE JB Weld, but it's awfully close....
There are also less-hard silicones out there that might be a bit more appropriate than the black stuff.