Just had a look at the manual here as Sue says it's fine for Gel, Looks like you have to take the case off to change the switch to Gel.
As for size of chargerrelitive to battery bank size it depends how yoiu use your van. Although in my Hymer I have 3 Gel batteries on the Leisure side I only have the standard 18 amp charge. The reason I've not upgraded the charger is because I ralely ever use mains to charge the batteries I rely on 300 watts of solar. Only likly to overload the charger if changing the batteries from flat.