I'd assume they're in the business of selling batteries, since just about everything I read says otherwise:
"GEL: The gel cell is similar to the AGM style because the electrolyte is suspended, but different because technically the AGM battery is still considered to be a wet cell. The electrolyte in a GEL cell has a silica additive that causes it to set up or stiffen. The recharge voltages on this type of cell are lower than the other styles of lead acid battery. This is probably the most sensitive cell in terms of adverse reactions to over-voltage charging. Gel Batteries are best used in VERY DEEP cycle application and may last a bit longer in hot weather applications. If the incorrect battery charger is used on a Gel Cell battery poor performance and premature failure is certain."
In particular are the voltage requirements; that's something that's fairly easy to control in a plug-in charger (simple design parameters to limit current and voltage), not so much so when you've got an alternator supplying the charge. You could probably get a lower limited voltage regulator to keep the electrical system within the ranges specified, but then we're talking about special charging/electrical systems, which was the point of what I said earlier.