I played through the entire game without picking up the Freeze Blast. Apparently you can pick it up very early in the game but it is very easy to miss. It can be found inside the Panessa Studio building where Batman keeps people in isolation cells. It is mainly used for Riddler Puzzles but can be upgraded to be very effective against groups of enemies.
I hope this helps you fill in your missing gadget slot! The other gadgets you will acquire during the storyline missions.