Holy moly they are expensive, is there anything reasonably priced? Say 100 quid?No good for a gel, assume you meant 8 amps. It will only charge the Gel to 80% not good for longevity.
For a Gel you need a bulk charge up to 14.2/14.3 then an absorption phase i.e. remaining at 14.2 v with current limited to about 1 amp for 8 hours then drop to maintenance of 13.8v.
Votronic & Victron do suitable chargers.
Edit:
Just looked at the spec of your charger it is just a maintenance charger doesn't have a bulk stage as such it is only 0.8 amp, it will take at least 3 days to charge your battery.
You really need a bulk charge rate of at least 18 amps.
At this rate it will be cheaper to buy a 100w solar panel for the garage roof and plug it into the Votronic duo in the van ?