Alright, so, assuming that we won't be seeing a Slowbro BREAK (not a 100% given, but probably likely), let's take a look at some possibilities, shall we? Slowbro is 014 and Gyarados-EX is 018, so there's 3 spaces in between, and a limited number of which Pokémon could fill those spaces. Almost all of the Water-type Pokémon in between Slowbro and Gyarados are 1 Stage evolutions: Seel/Dewgong, Shellder/Cloyster, Krabby/Kingler, (Horsea/Seadra, if Kingdra is a Dragon-type), Goldeen/Seaking, Staryu/Starmie.
So, here's what could fill those 3 spaces:
1) A Basic/Stage-1 Evolution line with a BREAK Pokémon for the Stage 1.
2) A Basic/Stage-1 Evolution line with no BREAK but a Magikarp, even though Gyarados is an EX in this version.
3) A Horsea/Seadra/Kingdra Evolution line with Kingdra as a Water-type.
4) ???
I might be missing something, but those are the 3 I thought up, and they're arranged roughly in the order of what I think would be most likely to least likely.