A fire and a fighting type Pokemon is good against steel.
Fight, Fire and Ground.
get fighting type Pokemon and four gym badges
Pokemon Ruby, Sapphire and Emerald are all Hoenn GBA games. I recommend Emerald, because it's newer, with better qualities and a different Champion who is somewhat easier to beat (The Water-type Wallace instead of the Steel-type Steven).
Get type coverage moves
with fighting type pokemon
Fighting and Fire types can easily beat Steel types as there moves are super effective on Steel.
Use a fighting, fire, or ground Pokemon.
The Petalburge Gym uses Normal Type Pokemon so you should take pyschic Pokemon or fighting type Pokemon if you want to beat them quickly.
Take a fighting type or train your Pokemon more
A Fire-type Pokemon can deal 2x the damage to Grass, Ice, Bug, and Steel-type Pokemon.
You need Ghost-type pokemon, Dark-type, or maybe Bug-type pokemon. Don't use Fighting-type pokemon.
Steel type Pokemon are weak against fire, fighting, and ground types. So if you want to defeat a steel type Pokemon or catch one, make sure you use super-effective moves that are weak against steel type Pokemon.