Control of your fireplace will depend on the protocol of the codes which control it. The Pronto can only send IR protocol codes, whether by IR or RF.

In theory, if you fireplace used IR protcols and you could get hold of these codes, then the Pronto could control it.

I'm not an expert on this, but I would of thought it highly unlikely that your fireplace manufacturer would use IR code protocol to control the fireplace if it is designed to work solely using RF.