Since you’re going to have to remove the cover again anyway, I’d recommend you replace it. They are cheap and it looks like the cover you have could be rusted enough to be leaking THROUGH it. The bubbled paint is an indication of that.

As for gaskets vs. RTV - I’ve serviced at least a dozen differentials (fluid change to replacement) and have used both gaskets and RTV. I’ve found what’s most important is the sealing surfaces are as clean as possible.

Allowing the diff to drain overnight if possible will keep the fluid that pools in the bottom from constantly dripping on the bottom, but you can also stuff paper towels in there to soak it up.

Using a scotch brite pad and spray lubricant works well to remove old gasket material and wiping them down with brake or carb cleaner just before assembly. Use compressed air if there’s any lint left behind by rags.

If you’re using a gasket, make sure the bolt threads are completely clean on the bolts and in the case - RTV is often trapped in the case thread bosses and can affect torquing. With a gasket, uneven torquing almost guarantees it won’t seal. The easiest way to remove it is with a plug tap/thread chaser and compressed air. The bolts should be available at any run-of-the-mill hardware store if it’s not worth cleaning them.

If you’re using RTV, follow the directions on the tube as precisely as you can. Give it at least an hour to set before adding fluid (overnight if possible).

As to brands - I’ve installed fel pro and Dorman gaskets and haven’t had one leak yet, regardless of the type of fluid. When it comes to RTV and this application in particular, I’m with tech2 - “the right stuff” is great - but any RTV for maximum oil resistance (typically black in color) will work.

Don’t forget safety glasses, especially if you’re using compressed air.