you will need to buy a vent cover, you can get them in woodies or any hardware store. If its been papered over just find the hole and remove the paper and put the vent cover over it
bear in mind some times the open vents can cause a nasty draft! esp if its an apartment, the wind howls through our vents!! We got vent covers in woodies that close over, so you can close them during the winter and open then during the summer. Its still saft for gas, but best thing to do is get a carbon minoxide tester anyway