Hi @Ashley-And-Maria0.
The guest doesn’t have to let you know they are using a gas burner, nor is it in your house rules. So they haven’t done anything wrong.... technically.
If they have been travelling and camping, this has probably been a more familiar means of cooking. Do you give instructions on arrival, how to use the cooker? Or leave pictorial instructions?
We cannot assume everyone knows how to use our appliances, especially if coming from different countries.
I would have been upset too, but we have actually handed over a portable butane cooker when after a severe storm, there was a loss of power. This allowed guests to boil the kettle in the short term till power came back on.
(However, the types of cookers can vary and mainly become unsafe when used with the wrong sized pots. There has been a safety recall in Australia on a number of portable butane cooker models, due to them combusting, generally with oversized pots causing the whole cooker base to overheat)