First, having 2 ground Pokemon on the team uses up twice as much experience and isn't much more useful than 1 ground Pokemon, so replacing anything except Torterra is a bad idea. If you need to replace one team member, then Torterra is the best one to replace.
With that said, I'm going to compare Gible to Torterra. Torterra is already fully evolved and pulling its own weight, and it will continue pulling its own weight for the rest of the game. If you drop that for Gible, then you get a Pokemon that's relatively weak and useless until it grows around 31 more levels.
And when it finally does evolve into Garchomp, it doesn't suddenly become some sort of unstoppable juggernaut of doom. Pseudo-legendary Pokemon aren't magic. (Alakazam, on the other hand, is magic) In fact, it might still be weaker than Torterra, despite having far superior base stats, because base stats don't decide everything. (and acid stats don't decide everything, either) Yes, Garchomp is stronger than Torterra at the same level, but there's no good reason why they should be at the same level. The thing that really limits the usefulness of Pokemon is experience, because wanting more experience requires grinding. So I'm going to compare Torterra and Garchomp when they're given the same amount of experience. A level 36 Torterra takes 171053 experience to grow to level 60. If you dropped Torterra and gave that experience to a level 17 Gible, then Gible will grow to level 52.
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And this is how strong each of them ends up. (assuming 16 IVs and 84 EVs in each stat and neutral natures)
84 Atk [level 60] Torterra Earthquake vs. 0 HP / 0 Def [level 63] Lucario: 188-224 (104.4 - 124.4%) -- guaranteed OHKO
84 Atk [level 52] Garchomp Earthquake vs. 0 HP / 0 Def [level 63] Lucario: 162-192 (90 - 106.6%) -- 43.8% chance to OHKO
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So Gible is worse than Torterra and you shouldn't use it.
