Read the user manual as the temp should be thermostat controlled were with gas there is always a pilot light burning when the oven is in use and when the set temp is reached the gas burner is shut off and when the temp fall below set point , the gas is turned on again and the pilot light starts the burner
by using the timer , when the time is reached the gas is turned off and of course you will have to relight it
if you do not have a user manual , download one from google or talk with a gas repair shop