I guess I can add in a few points to this topic now, though there probably is already enough.
First of all, I don't see why Gible or any of Ash's other Pokémon can't evolve at some stage, I mean just take a look at Quilava - a lot of people doubted it would ever evolve or be used again. And theres even Phanpy, which I think is a great example because like Gible, Phanpy was obtained (and well, hatched) near the end of Ash's Journey. Ash decided to let Phanpy go along with him to the Battle Frontier, and it eventually evolved. So yeah, I don't see why he couldn't take Gible along with him and see it evolve. Ash hasn't had another Dragon type, so if it did evolve - it probably would be a very big powerhouse.
But you know what - Garchomp, in my opinion, as a powerhouse doesn't really suit Ash. I just thought of this, but I think if he ever showed Gible to Iris she would want to trade. Just sayin'.