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?
Best Answer
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.