There is a fairly large difference between these two types of batteries.
The type of batteries that are commonly referred to as SLA or Sealed Lead Acid batteries are the AGM (Absorbed Glass Mat) type. This type of battery has capacity advantages over Gel batteries so they have become the industry standard that is used in electric scooters, bike, go-karts, mobility chairs, and other light electric vehicles. AGM batteries currently outsell Gel batteries by at least 100 to 1.
Gel batteries are sealed and they have a lead acid chemistry, however, they are not typically referred to as sealed lead acid batteries and more commonly referred to as Gel batteries. Gel batteries are still available for mobility scooters and power chairs which had them as original equipment and which have chargers for them. Gel batteries must be recharged with a battery charger designed for Gel batteries because any other type of charger, such as one designed for AGM batteries, will damage them.