Instructions
Go to the Wayward Cave, which is located in the Sinnoh region. This is the habitat of gible in "Pearl," and you need to take with you a Pokemon that knows flash and one that knows rock smash to completely navigate the cave and have the greatest chance of encountering a gible.
Catch a gible when it appears in a random encounter. You may want to catch two so that you can trade one of them. To catch one, throw any type of pokeball at it when it is your turn in battle. The lower the gible's HP, or hit points, the higher your chance is of catching it. Also, causing it to sleep will increase your odds of catching it.
Use a gible in future battles to raise its level. It will evolve into a gabite at level 24.
Continue to fight using the gabite. When it reaches level 48, it will finally evolve into garchomp.