We need numbers to help you out. 12v gel cel, what amperage does it say on the label it'll probably say 7Ah or 10Ah after the voltage. What kind of charger do you have exactly?
Generally, if it's a proper gel cell charger that matches the battery, it'll be 14 hours for the first charge.
As I thought, leave it attached for 14 hours or so, it will self regulate and reduce charge as time goes on. If there's a charging light or LED it'll go out when done.
If you calculate that the charger produces 5oomA (half amp) and the battery is 7 A. then 14 hours is about right.