It takes a very long time. First you have to have done the quests to unlock the entire Safari Zone (catch a Geodude and catcha a Sandshrew). Then the Warden has to call you and tell you about item placement. Then go to the Safari Zone and put the Rocky Beach in there. Go into the Safari Zone and get into the Rocky Beach area. Then you have to place 13 plains items (shrub, white flower, and red flower) and 17 peak items (small rock, big rock, mossy rock) anywhere in the area. Once you've done that, then you have to wait for 100 days! That's right, 100 days! With that configuration, Gible will appear after 100 days and that is the only way to get them to appear. And whatever you do, DON'T switch the Rocky Beach out of the Safari Zone. If you do, all the items will be reset and you'll have to start the 100 days again.
You can seep up the 100 days though. Set up the Safari Zone like I said, leave the Safari Zone, then save the game. Start the DS and change the time (DON'T CHANGE THE DATE) to 23:59 (or 11:59pm). It will ask you to turn the system off so do that. Turn it back on and load your game. Let the clock roll over to 00:00 (or 12:00am). Save your game and turn the system off. Turn the DS on and change the time to 23:59 again and repeat. Every time the clock rolls over it will add one day. Do this 100 times and Gible will start showing up in the Safari Zone.
Finally, level Gible up to level 22, and it will become a Gabite. Level the Gabite to level 48, and it's a Garchomp.
I hope I helped!