You should have 2 switches on the panel for the water heater. One for gas and one for electric. You can actually run both at the same time if you wish.. That will allow the tank to recover quicker than just on one or the other..
Since the electric part works, that's only when hooked up to 120v, you have the secondary switch turned on. That's located in the WH housing when you open the outside access to it.
From there, you should see the gas hookup. Make sure it's all clear of debris and all.
But, you should still have a second switch inside to turn just the gas function on...
Mine is a newer model, but I don't think they have really changed in the last 30 or so years...
Good luck! Mitch
