you mean the little inspection cover on the bottom of the bellhousing? is this on an auto trans vehicle? On my fj62 there is a inspection cover on the bottom in the 6 o'clock position, it allows you to get to the tourqe converter bolts, then there is another at about the 9 o'clock position if you are looking from the front, i think its for timing marks. funny, with all the s*** in the way I could not even get a timing light in the area.
On both I dontremember any gaskets, but if you are worried maby a bead of black silicone would help.
check toyodiy.com for parts diagrams and numbers.
the gaskets help seal the inspection cover. I bought from Toyo, but they're just some dense neoprene... and you still had to glue 'em down... Just get some generic from autozone, cut to fit, and use rtv or gasket adhesive to glue
my gasket seems to be long gone. I haven't noticed any problems because it's missing. You can do like Spike suggests, or, you can probably get a rubber strip with adhesive on one side from McMaster-Carr