The only way to catch a Gible in the wild in "Pokemon: SoulSilver Version," as well as its counterpart, "Pokemon: HeartGold Version," is by luring one out in the Safari Zone's Rocky Beach area. Fans of the franchise will remember the Safari Zone from previous generations, but this generation of Pokemon games offers the ability to customise the Safari Zone to attract new Pokemon that you wouldn't normally find there. The process for drawing out a wild Gible is not difficult but is very time-consuming.
Travel to the Safari Zone, west of Clanwood, and speak with the Safari Warden. If you have not already completed his two errands, bringing him a Geodude and Sandshrew, respectively, do that now. Completing these two tasks allows you to customise the different areas in the Safari Zone.
Return to the Safari Zone one day after you have brought the Safari Warden the Sandshrew. From this point on, you are allowed to customise the different areas of the Safari Zone using different objects. These are grouped by the type of area where you might find them, such as Peak objects, Plains objects, Forest objects and so forth.
Use the brief tutorial given by the Safari Warden to learn about customising the Safari Zone areas. Travel to the Rocky Beach area and go to the north of the large body of water, where you'll find a platform of grass. Walk just west of this area to the corner with the large stone, and place 13 Plains objects and 17 Peak objects in a cluster. These are necessary for drawing out a Gible.
Wait 100 days before going back to the Safari Zone. You can actually wait for 100 days, or you can alter the internal calendar in your Nintendo DS. To do this, go to the main menu and tap the DS icon at the centre of the bottom of the screen. Tap the calendar icon and change the date so that it is set to 100 days in the future. Save your settings, turn the DS off, turn it back on and go to your "Pokemon: SoulSilver Version" game.
Travel back to the Safari Zone and go to the Rocky Beach area. Position a Pokemon that is between level 30 and level 50 and spray a Repel on it to increase your chances of encountering a Gible. Go to the patch of grass near the northern part of the map, where you placed the Peak and Plains objects, and walk around until you encounter a Gible. Throw Safari Balls until you capture it.