I have had the side cases off my duc and a number of others and found that there is no gasket, even on new bikes. Ducati will sell you this white goop in a toothpaste tube. Its great stuff.
I use loctite ultra blue. a: because its better, b: ultra blue makes anything look like a race bike!! c: it's easier to apply and wont move about and get damaged when youre trying to refit everything. Only thing with goop gaskets is that everything needs to be 100% free of any oil and need a small curing time. You dont need a HUGE bead of goop, just a small thing smear is enough. If its messy, its because you have put too much on.
Gaskets are so 1950's man! dont waste your money on one. The goop is just as good, if not better. I hate reparing cars with cork rocker cover gaskets...some makers still sell cork for rocker covers.