Can you spray this inside a gas fireplace for wasps before using the fireplace?

Well, we don't know what IT is- you did not share that with us. However, many spray insecticides are combustible- they can burn. IF you used an insect spray in a fireplace, you should wait several hours before lighting a fire, and give the flue time to air out. Or you may get a large KAWHOOF when lighting the fire. Be sure that a large wasp or hornet nest has not blocked the flue.