However, you can evolve Onix into Steelix in Pokemon Gold. Here's how:
1. Find Onix: You can find Onix in the following locations in Pokemon Gold:
* Rock Tunnel: Found commonly.
* Mt. Mortar: Found in the area outside the cave.
2. Level Up Onix to Level 32: This is the required level for Onix to evolve into Steelix. You'll need to train Onix by battling other Pokemon to gain experience points.
3. Trade Onix with someone who has a Metal Coat: You can trade Onix to a friend with a Metal Coat, and then trade Onix back. This will cause Onix to evolve into Steelix.
Remember, the Metal Coat is an item introduced in Pokemon Silver and Crystal, so you'll need to find a friend who has those games to complete this step.