Your charger appears to be a basic car starter battery charger. I'm not that familiar with Gel batteries tbh as they aren't very popular.
According to one article I've just read, Gel and AGM batteries have different charging profiles. Then another article says that Gel and AGM have similar charging profiles.
?
My first thought would be to contact Ring in Leeds and ask them. My gut tells me that as AGM are much more popular than Gel, it would seem logical to me that they would indicate AGM on their charger instead of the less popular Gel if it was suitable.
My experience with basic chargers is that they will all charge most batteries to a greater or lesser level of charge. So if the charger's output voltage was 14.2v and the AGM needed 14.8v then the charger would still charge the battery enough to start the engine. Using the same charger over a period of time on an AGM battery would eventually 'damage' it.