The Gel battery is more delicate than the AGM as an over charge (even a short one) can bubble the Gel away from the plates lowering the battery capacity. This won't self repair.
The Gel battery has a couple of major pluses- (1)- it can be mounted upside down without issue, or remain in bike when it is shipped even if the crate ends up upside down for the trip.
(2)- the Gel battery has a very long shelf life without self internal discharge.
But-- The Gel battery is also more prone to sudden battery failure without warning.
The AGM battery is actually a better motorcycle battery as it better handles slight overcharging, also has a decent shelf life, & good deep cycle capabilities. PLUS the AGM doesn't die suddenly.
The downside of an AGM is it needs the alternator output voltage to be a bit higher than the Gel does so using an AGM in a motorcycle that came with a Gel battery might leave it slightly undercharged.
Neither a Gel or AGM battery is good if bike is parked for many weeks with battery connected to the bike's electrical system as the modern BMW motorcycle has enough onboard electronics to have a continuous electrical draw on the battery & will lower it's voltage as time passes.
On the other hand either a Gel or AGM will stay charged for a very long time (IF) the battery is disconnected from the bike's electrical system before storage.
For what you want to do (leave bike in storage for many weeks) then neither a Gel or AGM will work perfectly unless you disconnect the battery, or use a PROPER output battery tender to keep it topped up.
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