Instructions
Fight battles with a Grimer or Muk Pokemon. These Pokemon learn "Gunk Shot" at levels 44 and 54, respectively.
Level up your Ekans or Arbok Pokemon. They will learn the "Gunk Shot" technique at levels 41 and 56, respectively.
Catch an Octillery in the wild. There is a chance it will come equipped with the "Gunk Shot" technique.
Level up a Gulpin or a Swalot; they learn "Gunk Shot" at level 54 and 66, respectively.