Gible is strictly a HIdden Pokemon on Route 111. Only Hoenn Pokemon will appear as hidden until you defeat Groudon/Kyogre, after which Pokemon outside of Hoenn will start to appear.
If the Pokémon has a special move, Hidden Ability, high level, or high potential, an exclamation mark will appear next to that datum on the screen.
Source: Bulbapedia/ DexNav
You need the Go-Goggles to traverse the desert. You get it from May/Brendan after you obtain the Heat Badge (4th gym).