While nearly worthless on its own, gel can be monetized much more effectively (about 5 per 1000) with a Solidifier and 1 piece of wood for each six gel, by making Slime Candelabras. See the Making Money guide for full details.
Black Slimes do not drop their own color of Gel, unlike many other slimes.
The "tasty" part in its tooltip may be a reference to the fact that gelatin is often used in different foods.
Umbrella Slimes are the only source of Gel that is not guaranteed to drop it all the time, having only a 97.78% chance of dropping gel instead of a 100% chance. However, they are still high enough to successfully farm Gel.