I use a Rey Manual Therminder for temperature control on my 2 electric ovens. Bought them many years ago and do not remember them as outrageously expensive. A 115 solenoid to turn the gas on and off on yours sounds easy. No clue what to use if you are looking for more automation with various temperature stages and timing.
One oven we use for setting Loctite with temperatures around 180 to 220F, the other for occasional heat treating, both controls have worked without a hitch for all these years.