My limited experience of gels is that they make a right mess. I think I’ve only used them once or twice when I’ve been given them at Sportives, but I can only assume that those who willingly go out and buy them, throw the wrappers on a ride. You wouldn’t put yourself through the inconvenience otherwise. What a chore they are. Sticky hands. Sticky pockets. Any sportive organiser who gives them out for free should be obliged to walk around afterwards and pick up each and every one by hand.
Gells come out of a back pocket, and the wrapers tucked into shorts, ditto other food. Empty out shorts at the next bin/finish.
As for rules, in a strinct interpretation you can be DQ’d in a sailing race for littering, but it gets all knids of bizzare interpretations beyond that. For example some polishes are banned as they dissolve into the water, and before the days of asymetric spinakers which make leeward hois easy, it was normal to put wool ties arround the sail before hoisting, then a quick tug on the sheet and it would break them.