We get this often with surfboard bags.
Spray the puller with any WD40 type spray lube. Try to pick out as much of the salt/oxidisation crud as poss with a small sharp tool. Loop a cable tie through the pull tag for extra tug.
Pull (both ways), spray, pick, repeat. If and when it releases, spray the whole zip and run the puller up and down the full length of the zip a few times.
Be carefully not to break the zip puller off, been there.