I know that In most games onix evolves with metal coat and a trade but I saw that in twitch plays Pokemon they managed to get a Steelix so I was wondering if there was some other way?
There's no other way on your regular Gamboy game. You must trade Onix holding a Metal Coat in order for it to evolve.
In TPP, they used an emulator which allowed them to get every single Pokemon available in the game, meaning they didn't have to trade in order to evolve Onix.
TPP Plays Crystal - Onix evolves