I've had my toilet off in order to reseat my black vent pipe into the smaller grommet. I removed the toilet, then the floor flange. The 3" pipe going into the toilet comes out with the flange. You will see the 3" grommet. Mine has a split in it, which I didn't fix because we were leaving in a couple of days. I don't get any liquid out, but I do get some smell.
I have one on order, along with one for the vent, and will replace them after next trip.
I believe the grommet has an outside grove in it that the tank top hole fits in. I didn't see any sealant. I think when the pipe is replaced it is a firm seal.
Best to do with an empty, flushed tank