In competitive, Gliscor and Garchomp have different niches- Gliscor being a physical wall while Garchomp is a physical attacker- but in-game, where defense is not needed, Garchomp does everything that Gliscor does better.
- While both have access to Earthquake as their Ground type STAB move, Garchomp has a slew of powerful options for its Dragon type STAB such as Outrage, Dragon Claw and Dragon Rush, while Gliscor's most powerful Flying type STAB is the weak Aerial Ace. Because of this, Garchomp can hit more things with powerful STAB moves than Gliscor can.
- Garchomp, having high Attack and Speed, has much better offensive stats than Gliscor. Although Gliscor has nice Defense, that really isn't useful in-game because most Pokemon won't hit you very hard anyway.
The only reason I could think to use Gliscor over Garchomp is for Cynthia, because between Roost and Swords Dance Gliscor could setup on Spiritomb and then sweep. But this role could be delegated to a better Pokemon like Scizor, who also has access to both Swords Dance and Roost, and resists Spiritomb's STAB moves. Plus, this strategy isn't needed to beat Cynthia anyway.