I don't think you need to remove it. I think I've planted fresh seed from supermarket tomatoes before. I also made a silly mistake last year and composted some finished tomato plants (presumably at least one had a little unripe or split fruit still on it) and have been picking tomato seedlings out of the compost all year.

OK, thanks, that's a relief! They do seem to have quite a life force, don't they?

I have been harvesting seeds for years from the same tomatoes. Yes, you must remove the gel. First soak the seeds for 24 hours in warm water. Then strain them and lay them on a paper towel. Very gently push them sideways to rub off the gel. Rinse. Place them in a single, well spaced layer on the shiny side of a piece of freezer paper. After 48 hours turn each seed over and dry for another 48 hours. They should be bone dry. Seal in a paper envelope and store until spring. I know this sounds tedious but that gel contains chemicals that inhibit growth and must be removed. I have a bout a 95% germination rate.