You can get Gible in the Safari Zone in Pokémon HeartGold in the Rocky Beach area however it'll only appear there after you have placed 13 Plain Objects and 17 Peak Objects there and after you have waited 100 days.
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Gible is not available in this Pokemon version in the wild. to acquire Gible, you must beat the Elite Four and transfer this Pokemon through the Poke Transfer Lab located on Route 15. You must have Gible in your PC in another Pokemon game used on a DS (Diamond, Pearl, Platinum, Heartgold or Soulsilver.) This is true for all Pokemon you want/need in previous generations.
Garchomp is obtained by evolving Gabite. Gabite evolves into Garchomp at level 48. To get a Gabite, you must first have a Gible. Gible evolves into Gabite at level 30.
There is no other way unless you want to trade
Safari Zone (Rocky Beach Area: If Grass and Rock Objects are placed, may appear in Tall Grass)
Gible doesn't exist in sapphire.