Are you talking about the middle of the J section of the trap where it swivels? If so, usually there is no gasket there. Just need to get in aligned properly and DON'T over tighten. Mine broke at the first nut just below the shower drain tailpiece.
When I repaired them both in the kitchen and the shower I installed a (Fernco) rubber coupling to on the vertical section just below the tailpiece to take the stress off the pipes when stepping into the shower and driving down the road.
My Kitchen sink is on a slide so I also (after a cracked fitting in the same place) Used a Fernco coupling on the horizontal section to also keep stress off the section of pipe. You can get them at Lowes, Home Depot or any Plumbing Supply.
Notice also I keep an (Just in case) pan under the drain and check it once in a while.
Also nice that it keeps you from having to perfectly align the pieces together.
Also notice the Access panel I installed in the hallway. The small opening in the bathroom is a joke to get at anything to fix.
Comes in white and painted it brown like the walls.