No good steel STAB for Magneton sorry :( Here are some possible suggestions, choose the ones you like best:
Thunder Wave: Paralyzes the foe.
Lock-On+Zap Cannon: Takes two turns, but paralyzes the foe while doing high damage.
Swift/Return: Doesn't miss or higher power.
Thunder: This is the most powerful Electric type attack you are going to get in Gold because of no Thunderbolt TM.
Supersonic/Swagger: Confuse the foe. Choose the one you like best.
I'd go with: Thunder, Thunder Wave, Swagger, Return.