The voltage thing can be a problem depending on the amperage going into the battery and the size of the battery. ie car batteries charge at about 14 to 14.5 volts and varying amperages but if you were to charge it at 18 volts and only 20 watts of power, there would be no problem at all because of the size. It wouldn't hurt the battery at all but if you charged the same battery at 18 volts and say 25 amps, it would burn the battery up.
When you are checking on gell cells, you have to be a little careful. They can start to solidify near the plates. It will put out full voltage but the amphour rating can be less than a third of new. They make or at least made gell cells for atv's. At the price you are talking about, it probably wouldn't hurt anything. Just check near the posts to see if it starts to swell or ooze anything. Gell cells are not quite as tollerant to the same types of charging that a regular wet battery is. The normal gassing during a charge has nowhere to vent and has to be reobsorbed by the liquid acid. If your current battery seems to spatter a little out the drain tube then a gell cell may not be good for you. On the other hand, if the rating is a little higher than what is standard, then you have the extra area so to speak to obsorbe the varying power.
On the UPS thing, it is uninterupted power supply and they are in all kinds of things from cash registers to phones to the emergency lights in buildings (the ones that look like there are headlights at the top. I wasn't really trying to scare you away. The power that an older moped gives is not that much. You might could go with a gell cell just fine. If you are wanting a back up though, the JC Whitney battery would be a better set on the shelf type of a thing. Whenever you need it, just put the acid in and charge it a little. Next week, next month, next year. With a gell cell or any activated battery, just sitting is bad for it. If you want to learn more than you really need about batteries, check out some of the web sights for alternative energy or solar power. They are about 30% batteries. They have a new type of battery out that uses a glass webbing that works like an electrolite starved battery except that the glass webbing sits against the cells of the battery for the electrolite contact. Anyway, try the battery out if you want, just keep an eye on it. Like I said earlier, it cannot vent. Thats what makes them good and that is also one of their weeknesses. If you always drive with your lights on, you probbably wont have a problem.