I am playing Pokémon HeartGold, and I have a Gyarados and a Steelix. I had originally planned to use Earthquake on Gyarados because it covers all of its weaknesses, but then I realized that I didn't have a good STAB move for Steelix (other than Dig).
Maybe I should give Gyarados Dig?
Steelix sounds like the most logical since its a steel and ground type Pokemon. However if Gyarados knows earthquake, that gives the element of surprise.
In my opinion, Gyarados seems like the ideal choice. Gyarados is a water and flying so having a ground type move to counter electric types. Hope I helped
I'd say Steelix.. He doesn't really have a good movepool at that time
A simple answer: both. Both Gyrados and Steelix require Earthquake. You can never have too many Earthquakers in a team. Remember that. However as you are playing HeartGold, you'd have to buy a second one in the Battle Frontier shop for 80 BP. I f this is not possible I would suggest Steelix having Earthquake, he needs it.