Hello: There used to be a timer called “Quarterback” or something similar. It was a control that mounted directly to the gas valve and adjusted it up and down daily. The problem I found using them is the gas valves are not made to be used so much and I got small gas leaks from worn out seals. Most of the same benefit can be had by insulating the tank and piping very well. That way there is nothing to break 😉
A study was done years ago to measure benefits of setback on electric heaters and found minimal savings with daily setback. I know it would be different with gas heaters, but any conservation efforts must be weighed against safety. Water temperature and Legionnaires disease is another example.