I have had a look in the books of monumental inscriptions for St John's kirkyard in Gamrie and Doune kirkyard in Macduff and there are no inscriptions there which help. Several Joass/Joss which are probably ancestors, but no Joass/ Barron.
Have you found Walter's death certificate to get his father's name? Do you have him on a census giving his birthplace as Gamrie?
You may have to follow up both possible families. If you can get hold of the microfilm reel for Gamrie baptisms and check all the various witnesses it might give a pointer. If one of the two families shows as living in Gardenstown or Crovie with the father a fisherman, it would be more likely than if the family was working the land. If Walter was still in Gamrie in his 20s, he might appear as a witness to baptisms.
Although there are no gravestones to Joass/Barron at Gamrie, the burial register was well kept from about 1822 to 1854. Also the marriage entries for Gamrie can give more detail than usual. Often there are two entries, one in the Gamrie register and one in the Macduff register.
Graham.