How to open a 1976 Porsche gas cap.  It will unlock but will not open. ?

The cap I had unlocked when you put the key in and turned it 1/4 turn and back to remove the key. Then the cap is unlocked and will counter clockwise spin out. When you are tightening the cap back on, when it clicks tight it will automatically re lock the gas cap.

If the cap is defective, drill a 1/8 inch hole through the cap next to the lock cylinder and leave the drill bit stuck in the cap. Now the cap will spin off as the 1/8" drill bit will connect the inner and outer part of the cap to remove it. Then you'll obviously need a new replacement cap if you drill it.

You didn’t state which exact Porsche model, but I’m unaware of ANY Porsche sold new in 1976 which came with a locking gas cap from the factory, and to this day they’re all sold with  just a locking gas cap cover instead of a locking cap.

So what you have is an aftermarket part fitted some time during the last 43 years or so. 

Without crystal clear photographs it’s going to be impossible to offer much useful advice. If you have definitely now unlocked it but cannot remove it, then the cap is probably going to need brute force and possibly tools to remove it. Drive to the nearest repair shop if you’re seriously low on fuel or to your preferred repair shop if your fuel load allows.

Did you try pressing in on it?

Porsche gas cap will not unlock. We are not, I repeat NOT, going to help you steal a car.