Like most Pokmon cards, the value of Gulpin depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Stellar Crown)
Most of the time whenever a One Piece Card Game set releases, there is a shake-up in the meta for the first couple of weeks, but then players get comfortable and return to their favorite decks
Break the Bank with Final Fantasy Lets have a closer look
As one of the first starter decks that came out, he's aged quite well, but he has aged and that's what keeps him from being in A tier
Cornerstone Mask Tera Ogerpon ex (119/101) This card has the same Attack and Ability as the Cornerstone Mask card listed previously, so theres not much else to note about that
As with the first games, you don't get it post-game