As you progress your game in Story mode, you unlock most of the Batman's gadgets. But Freeze Blast is a Gadget that will not be unlocked with mission or side missions. It's a Hidden gadget in Arkham Knight. Here is the guide how to get the hidden gadget 'Freeze Blast' in Batman Arkham Knight.
What does the Freeze Blast do?
Freeze Blast is hidden weapon which can temporarily immobilize the enemies. You can use it in a middle of a combat to disable them for a short time and you can hit them or run away.
Where can I find the Freeze Blast?
The Freeze Blast is hidden in the Bleake Island near Panessa Movie Studios. You can use the elevator to go down to the Quarantine cells. Once you reach down in the cells walk forward to the big open room where you will find one more room at the left side. The Freeze Blast is on the table next to the Fire extinguisher.
Hint: Use your Detective mode you can see it glowing.
Hope this guide was helpful. If you know of any other special new Gadgets in this final Batman: Arkham game you can comment below.