The only thing the gasket does is keep dirt from getting into the diff case. I tried running without it once, but didn't like the result.
You can use just about anything in place of the gasket. I was in a pinch when I was racing my OD truggy, and used two layers of blue painters tape. Worked rather well. Just make sure you trim out the area where the chassis is recessed, or the ring gear can grab the tape.