Electric kitchen devices, such as microwave ovens, almost always have a beeper fitted, to let users know that the heating/cooking cycle is over.
Of course, this is trivial, because they work on a timer in the first place.
In the other hand, fossil fuel devices, such as ovens and heaters, do not have timers, and, chicken and egg, alerts. Overcooked food is the subject of many jokes.
However, untended gas cookers are a home fire hazard, apart from wasting fuel and money and creating climate change effluents unnecessarily (meaning, while not even providing the benefit of cooking).
A simple, and hopefully inexpensive, heat sensitive device, that might detect an open flame, or might detect that a flame has been burning in excess of some ‘normal’ time, could be very useful.
A set of bimetallic elements could be used to detect the heat of burners that have been lit, and this could be linked to an electronic timekeeper to sound off an alarm, which could be a buzzer or beeper. The power source could be batteries or mains power.