I've finished the game (main story at least), but there's still one slot that's locked on my gadget unlock screen. There's also a spot left open to the top left of my gadget selection menu AND one physical Riddler challenge that's locked because I don't have the right gadget.
Apparently, this gadget is called a Freeze Cluster.
How do I unlock the Freeze Cluster?
The second time that you go to the Steel Mill, un-gag Harley(you can always just re-gag her afterwards ;)), and she'll tell you about stealing some of Freeze's stuff. Follow the objective marker, and you should get the Freeze Cluster pretty easily.
It can freeze multiple people near each other, immobilizing them, but unlike the normal Freeze Grenade, it won't keep them from shooting.
The Freeze Cluster is unlocked by completing the side mission "Hot and Cold" given by Harley Quinn in the Steel Mill after you ungag her. She's only there the second time you go in.
The mission hot and cold brings you to an elevator, go on top of the other one and use an electrical charge to move the other one up and go down through the gap.Once there take out the guards and the other enemies, you'll find the freeze cluster than.
