What is the last gadget in Batman Arkham City and how to unlock?

[James]

I have beaten the game and going back through getting everything else, but I still have one gadget box to unlock- what is this and how do I unlock it?

The gadgets I have are:
Batarang
Remote Controlled Batarang
Batclaw
Explosive Gel
Cryptographic Sequencer
Smoke Pellet
Remote Electrical Charge
Line Launcher
Reversal Batarang
Freeze Blast
Batclaw Disarm
Sonic Batarang
Sonic Shock Batarang
Line Launcher Tightrope
Freeze Proximity Mine
Cryptographic Range Amplifier
Cryptographic Power Amplifier
Disruptor Firearm Jammer
Disruptive Mine Detonator

So, Im missing the last bottom right hand box. Any help or info is greatly appreciated.

 

[mr_j526]

It sounds to me that you are missing the Cluster Freeze Blast, which is gotten inside of the Steel Mill. When/if you ungagged Harley during your second trip into the mill, she should have mentioned the Freeze-tech that was sent down to the basement. Down there you will find a small crate containing the last gadget.