Gel cells will tolerate a certain amount of abuse. The voltage regulator on my boat alternator went bad & was charging my gel cells at 18V. They got a total of at least 10hours charging at this voltage with no noticable degrading. They lasted another 5 years after the fault happened.
Yeah, yeah, we have heard it a thousand times, a couple of 6V golfcart batterys provide the best bang for amperage buck. What these drum thumpers don't say is that they have to replace those wet cell batterys every 3 or 4 years. Expect 8 to 10 years service life out of a gel or AGM battery.