You might not even need to get a heat gun, if you have a blow dryer that is what I use after thawing to warm them up a bit.
You don't want to heat it up unnaturally hot or it can deter your BP from striking. I try to get the head of the mouse just up to about 85-90 degrees.
In order to take the surface temp of the mouse I would recommend a IR thermometer such as:www.tempgun.com/