Gel batteries are generally the same as regular lead-acid batteries you use to start your car, except the battery cells contain a gel rather than a fluid. Gel batteries are considered to be safer, because if the casing gets damaged the gel doesn’t spill, but fluid-based batteries can leak sulfuric acid, if the casing gets damaged. Gel batteries are sealed units, so unlike fluid-based batteries you cannot access the cells. The best way to fix and restore a gel battery is to discharge it as much as possible and then charge the battery slowly.
Check your battery is discharged before fixing and restoring it. Turn on your car’s lights and other electrical equipment to use any power in the battery.