Not all tanks have drain plugs. I found some do, some don't. If yours does, make sure to spray rust penetrant around it, let it sit, and then use an impact wrench on it. I've found some drain plugs to be really frozen in place, and the metal of the tank is so thin I worry about damaging it by torquing on the ratchet too hard.
For the fuel gauge, remove the sender from the tank (careful, the screws are probably very rusted) and clean its contacts. It's just a float that moves an arm up and down a strip of metal which alters the resistance that is sent to the gauge.
Farmers always seem to have a use for old gas. If you do decide to drain the tank, post an ad for free old gas on craigslist or whatever platform is popular in your area. I bet a farmer will take it. Some of the recycling facilities around me accept old gas and such but you have to call around to figure out which ones do and what their requirements are for dropping it off.