No time for a dealer visit?
Get some RemOil with the long red straw attached to the spray nozzle and spray on the right-side where the latch-grab exists. Keep pushing inward after a few bursts of oil, until it frees the door. Then grab a rag and wipe down the inside of gas cap area. Maybe add a squirt or two to the hinges too.
I always use rustproof-type gun oil on parts like this. Never use stuff like WD-40.
No gun oil?..... try silicone spray.