How do you get a gulpin?

Answer 1

You do not have to follow me word by word, but here's what I would do.

In Ruby, Sapphire, or Emerald:

Equip a team of Ghost, Ground, and Rock Type Pokemon. They are only to resist against Gulpin, and might not launch super effective attacks. Be sure they are around Level 13, so that Gulpin will not suffer a one-hit KO. Head to Route 110 and scan the tall grass. Gulpin are rare but you will find one eventually. Send out a lead Pokemon, one with average strength but enough to stay in battle. Weaken Gulpin with your attacks, and, if possible, inflict it with a Status Condition. Throw a Pokeball of any type (but please, please, PLEASE not a Dive Ball). If not caught, weaken Gulpin more and try again.

In Pokemon XD:

Go to the Shadow Pokemon Laboratory. A Gulpin will be found there.

In Pokemon Diamond, Pearl, and Platinum:

Head to the Great Marsh after obtaining the National Pokedex. Use the binoculars to find Gulpin, then head out into that area. Use the bait and mud to prepare it for capture. Throw a Safari Ball.

In Pokemon SoulSilver:

There will come a time when a swarm of Gulpin arrive on Route 3. Equip your team with Ghost, Ground, and Rock Type Pokemon around Level 5. That way the Pokemon may resist Gulpin and launch decent attacks, but not inflict a one-hit KO. Head to Route 3 and search for a Gulpin; it won't take too long. Weaken the Pokemon, and possibly give it a Status Problem. Throw a Pokeball to capture it.

Gulpin can only be traded to the other Main Series games. Gulpin is not obtainable at ALL in Generation I and II games. Good luck!