The gasket should be replaced. The simplest and most commonly used solution these days is to replace the entire gasket with the appropriate size of adhesive-backed weather stripping. A better, but more time consuming solution is to replace it with some very soft leather, e.g. something like chamois texture. It should have a smooth soft nap side exposed after attaching it. The problem with weather stripping is that some foam rubber will crumble after aging.
Many folks use it anyhow.
How to Apply New Bellows Tape Use ordinary white glue for putting on bellows tape. The standard technique is to spread a bunch of the glue out on a board. This gets just the right amount of glue on the tape. Then apply the tape to the bellows. If you are using ribbed bellows tape, be sure that the diagonal ribbed pattern is going in the same direction from one fold to the next.