Here's why:
* No Post-Game Content: FireRed (and LeafGreen) are faithful remakes of the original Red and Blue versions. They don't include the expanded post-game content of later generations like the Battle Frontier or the ability to battle rivals after the main story.
* Focus on the Original Story: The games aim to deliver a streamlined experience focusing on the core story of becoming a Pokémon Champion.
What you can do after beating the Elite Four:
* Catch the Legendary Birds: You can still challenge Articuno, Zapdos, and Moltres for a chance to capture them.
* Complete the Pokédex: Capture all the available Pokémon to complete the Pokédex and unlock the National Dex (which expands the number of Pokémon you can obtain).
* Train Your Team: With no further story events, you can spend your time training your Pokémon and challenging other trainers.
* Trade and Battle: Trade Pokémon with friends and battle them for bragging rights.
While Gary isn't available in FireRed after the Elite Four, remember that he's a major rival throughout the main story, and you'll get plenty of chances to battle him!