Well, x4 Grass weakness or x4 Electric weakness. I would actually go with Omastar because Gyarados got a huge attack stat but no STAB moves that get use of it. Omastar got a great Sp.Atk stat that Omastar get use of.
Do I have to put Movesets? I'll try my best
If you want Item I would go with Leftovers which is easy to find in Pokemon red version
Surf: I guess you will use this Pokemon as a surf Pokemon and Hydro Pump sucks in-game because only 5 PP
Ice Beam: The most usable Ice type move for Omastar If you dont want Blizzard
Seismic Toss: Does Damage Equal your level = usefull
Toxic: Stall them with your great defence stat
Hope you like it