AGM batteries are built differently than open lead acid batteries. In AGM batteries there is a fiberglass mat between the plates that is saturated with acid/electrolyte. This mat is then packed between the plates and compressed. This ensures a super-fast reaction between the acid and the plates. Because the sheets and mats are tightly packed, they are almost immune to vibrations.
This type of battery is maintenance-free, in normal use there is no gas formation. The little gas that would still occur is returned to the electrolyte, so the batteries should never be refilled. Also no ventilation is needed anymore, for safety there is an overpressure valve on the battery. The batteries can be installed in any position, except upside down.
Due to the high production cost these batteries are a lot more expensive than open lead acid batteries, but still cheaper than GEL batteries.