Can a Smart light switch be used for a Gas fireplace insert?
Probably not. On the gas fireplaces I have installed the on off switch was a millivolt contact. If this is the case there will be no power to run the smart switch unless it is battery powered. 
If the fireplace is a 120v control then in most cases you will need a neutral.  If you provide the model of the fireplace and the smart switch we could provide more information. There are some smart switches that do not require neutrals but depending on the type of control for the fireplace even those might not work.