Any reason Gulpin is on the list? I admit it's not very good, but I don't think you're a complete embarrasment if you try to use it. Might be because Trubbish is better in what Gulpin can do.
Gulpin is on the list because it's heavily outclassed by other Sticky Hold users and Poison-types. Trubbish, as you mentioned, is a much better choice overall, as it can restore its own Berry Juice with Recycle, can set Spikes or Toxic Spikes to capitalize on forced switches, has access to Drain Punch, and has a better Speed stat, all of which give it an actually usable niche in the metagame. As a wall with Eviolite + Pain Split, Gulpin is outclassed by Koffing, which has great physical bulk and Will-O-Wisp to cripple physical attackers and wear stuff down, and isn't easily trapped and eliminated by Diglett thanks to Levitate. Even if Gulpin tries running an offensive set, or like, a boosting set with Stockpile or Curse, you're better off with Grimer, which has a much better Attack stat, as well as comparable bulk and coverage options to work with. Grimer itself is pretty mediocre anyway; there are straight-up better offensive Poison-types to choose from, such as Gastly, Skrelp, and Croagunk, which offer more immediate power, secondary STAB, and several other advantages over Gulpin.
The only semi-notable thing Gulpin has over any of the previously mentioned Poison-types is access to Encore, but this is mostly done better by Cottonee, which has Prankster. If Gulpin ever gets a set, I expect Encore to be included, as it's the only potential "niche" it really has.
