@Zander
I will admit gyarados is the better sweeper due to attack power, 125 atk is much better than 90 sp atk, as well as in terms of how easy it is to set up. The only areas venomoth does better in are: having access to free hits via sleep powder, giga drain for healing, and having tinted lens to prevent bad type match-ups from ending the sweep. While these are great traits, they still don't hold a candle to gyarados's higher damage output and ease of setting up due to its bulkiness.
I'll also admit that I did a poor job of trying to make my point, as I focused more on venomoth, and less on the idea that the game could already be broken. As to addressing how the game can be broken, I will give some examples of how venomoth and crobat already do this.
Firstly, using venomoth I have managed to partially or fully sweep Corey, Shelly (once), Aya, Noel, and Radomus. Noel was very easy, as I was easily able to buff venomoth by using sleep powder to cover him. Radomus was a lot harder due to having the type advantage, but I still managed to take down 4 (almost 5) of his pokemon using only venomoth and crobat. As for Crobat, I was able to take ~85% of the giant steelix's health with crobat (think lax incense, confuse ray, and lots of double teams). Using the same tactics, and poison fang, crobat was able to take down ditto arceus by itself (Did this in 3-4 tries).
Based on these cases, the game can already be broken, so adding gyarados at this point probably wouldn't be too much. Now, if you really want to worry about really breaking the game, I would worry more about getting the ability to mega evolve or having the z crystals.