in surprised you cant hook up some wire to the ground GPIO and another one say GPIO18 and make GPIO18 high.
This is unwise, without knowing that the voltage that the key fob runs at is exactly 3.3. If it's different, you're liable to fry something by doing that. You instead want a method to electrically bypass the button, simulating a button press, based on a small input voltage from the Pi.
I'm not familiar with the "reed relay" that Paul suggested, but other methods include using a transistor or a relay to trigger the button circuit. You will find plenty of tutorials about using them for exactly this task on Google or YouTube.