To gas MANY pinky mice/rats toss them in a 1gallon freezer bag, Put co2 hose in and close the bag best you can. Turn it on and leave it run few mins then hurry and close bag.

If your freezer can get COLD and i mean below -20F you CAN freeze alive as they will die in about 30 secs and be rock solid in about 20 mins depending how many are in the bag.

Ive done both methods depending on amount i was gassing/freezing