The maximum for gel batteries is 14.1 volts. That can be temperature compensated, so when they are colder then normal it can be a bit more and when they are warmer it needs to be lower. Gel batteries are sensitive to overcharging so you need to get this right. If something isn’t working correctly the first suspect is always the temperature sensors, if you have these fitted disconnect them and try it again. The next suspect is that the voltage sense wire of the regulator may not be sensing the actual voltage, maybe because it is sensing from a battery that is not the one that is being charged, or maybe because of a bad connection. You can use the display on the regulator to troubleshoot,