Can Discover VRLA DRY CELL AGM and GEL batteries be installed in sealed battery boxes?

No.  Never install any battery in a completely sealed container. Although most of the normal gases (oxygen and hydrogen) produced in a VRLA DRY CELL battery will be recombined as described above in “How does a GEL or AGM VRLA DRY CELL battery work,” oxygen and hydrogen will escape from the battery in an overcharge condition. For safety’s sake, these potentially explosive gases must be allowed to vent into the atmosphere and never be trapped in a sealed battery box or a tightly enclosed space! Under normal conditions, GEL and AGM batteries will produce little, if any, a dangerous gas. Heat, however, can be generated during the charge and discharge process. To ensure the longest life for the battery, this heat must be allowed to dissipate as quickly as possible.