Tested with Openblocks Block Breaker, the Terrain Smasher from Thermal Expansion 4, and the breaker from Minefactory Reloaded. The gas tanks do not drop on a "single pulse" breaking method.

They do, however, drop when put next to an autonomous activator from TE4, with a pick inside of it.

Have not tested this with the Thaumic Tinkerer dynamism tablet, but I figure that would probably work like the activator.

This behavior is present in all Mekanism machines - they can only be dropped by a player breaking them. this is why the autonomous activator works, it simulates a player breaking the block.