To get Gible, you need to place 13 Plains Objects and 17 Peak Objects into a Safari Zone Block location. After that, just wait 100 days (Yes, 100 days...) And Gible will appear for catching.
They'll always be at Level 44, so it won't be long after you catch it before it evolves into a Garchomp! This process is long and tedious though, so if you have the means to do so, just catch one from Diamond, Pearl, or Platinum, and trade it over.
If not, though, get ready for a boring road to Gible...