There are 3 Heavy Gauntlets in the game.
#1: Cell Blocks. Once you've finished following Penguin's blood trail all the way down to a large gate, scan and pull down a wooden panel in the background.
#2: Industrial. After defeating Solomon Grundy, follow the linear path until a vent. After exiting the vent, pull down the vent cover on the wall in the background, to then enter it and follow the path. In here, disable the large fan with a batarang to then access this upgrade.
#3: Administration. After deafeating Joker, head back to the burning room. Go right, and use the tightrope to get up in the middle of the long hallway. Hack the Gamma-encrypted panel, which disables the gas and opens the gate to this final upgrade.