ask for help to the doorman of the building, usually he will give the best way to do it, also take into acount if there is an extra key to let the gas in just for the oven if it is not part of the stove
The valve at the wall is called "llave (valve) de paso". Argentines usually close it when they leave for a few days or some even before going to bed. It is a precaution just in case there is a leakage or malfunction of the oven. And also it is the valve you close whenever the oven needs maintenance.