I agree with you, when I did my upgrade, I removed half the case, and placed the aluminum half in place, flipped the case over, and did the same on the second hald. I didn't touch the grease or gears. My situatition is a little different because I have not run the truck yet. But I don't see why the gasket would need replaced just because the differential case was opened up. The gasket is in between the gears.

Now if you were playing in the mud and water, and the differential case got busted, Then yes, replace the gasket, clean the gears, then use clean new grease, and a new gasket. Nowhere does it state to use a new gasket, just that you must allways reinstall it. The way I read it, is you use the same one, after all it is rubber, and should last a very long time.