Is there a button on the inside of your 1989 jaguar xj6 to relese the gas lid?

Yes. Open your trunk lid and on the left hand side (the side of the fuel filler cap) there should be a label that says manual fuel door open. The switch is white and close to the top right by the fuel filler cap on the inside of the trunk. You may have to pull back the carpeting a little to find it as it might be tucked behind. Give it a pull and the fuel door should open. Best of luck