For the gas, get a bottle of gasoline stabilizer ("Stabil" is probably the most common brand). But you will be better off just driving it regularly. Perhaps you can just work out a schedule - I'd probably drive it at least once a week and at least about 20 miles per trip. Short trips would be worse than leaving it sit.
I would make my next regular drive be to your best locally owned, independent "radiator shop" - or something like that. Many areas have shops that do specialize in the cooling system. What you describe can have different kinds of causes. A good shop will find the cause and fix it. You may be looking at something as simple as a bad thermostat and/or radiator cap; a little more expensive like a clogged radiator or maybe that "new" water pump is already losing its fins; and then there is the worst case which is a head gasket.
Look at your oil - on the dipstick & under the filler cap. If the oil looks something like a chocolate milkshake AND you are willing to dump some $$ into this vehicle then don't drive it to a shop - have it towed. If you don't want to spend any $$ on it, sell it. There are plenty of people who would buy something like this cheap, fix the head & drive on. Don't expect to get much in that case.
But don't start assuming the worst case scenario.