During the first season of Pokémon, the writers bent over backward to make Ash Ketchum the biggest loser they possibly could. As if disobedient Pokémon and ineptitude in gym battles weren't enough, Gary Oak would also stop by every now and then to windmill dunk on the aspiring Pokémon master, to the delight of an actual cheerleading squad. Ash's Kanto Gym challenge is notorious for his questionably "earned" badges, but Gary's gym challenge is famous for his collection of 10(!) badges.
From Kanto to Kalos, the rules are the same: earn eight Gym badges for the right to compete in the Pokémon League. While Ash barely got all his badges in time for the Indigo League and didn't even earn most of them, Gary was apparently doing laps around Kanto defeating Gym Leader after Gym Leader just for funsies. Granted, it's much easier to traverse the sprawling Kanto Region when you have a chauffeur, as opposed to Ash's nomadic wandering with his friends.
It's long been an unspoken idea in the world of Pokémon that the eight Gyms that Ash visits per region aren't necessarily the region's only Gyms. It would make more sense that there are more than eight to give trainers some options -- otherwise, they may find themselves arriving for their challenge only to put their name on a waiting list. Considering seven of Gary's badges are not from any of the Gyms Ash visited, this would have to mean there are at least 15 Gyms in the Kanto Region. The idea that Gary explored much more of the region than Ash did would also explain why he has so many more Pokémon than our hapless hero.
The whole point of Gary as a trainer is for Ash to have a formidable rival. Unfortunately, Gary's talent is so over the top that it makes Ash look even more incompetent than he manages to look on his own. Ash only traditionally earned three of his eight badges in Kanto, while it can be safely assumed that Gary, thanks to his skill, rightfully earned all 10 of his badges. It's fair to point out that Ash made it farther in the Indigo League than Gary, but undeserved success is a main character's privilege, not his semi-bully of a rival's.
As for why Gary would opt to collect 10 badges despite the requirement being only eight, his entire personality makes the answer pretty obvious: he likes to show off. After all, he travels the region in a red convertible accompanied by a cheerleading squad. Gary Oak does not know the meaning of the word humility; he's much more interested in humiliation, particularly Ash's. From the size of his Krabby to the size of his entire collection of Pokémon, it's "go big or go home" for the grandson of Professor Oak.
Like many of Ash's other antagonistic rivals, Gary eventually concedes to Ash's method of bonding with Pokémon rather than treating them as interchangeable tools. He's long since given up the car, the cheerleaders, and the contempt for Ash, now focusing on Pokémon research like his grandfather. That said, it would be interesting to see him try his hand at Gym battles again in the future, and perhaps he could even show Ash these mysterious, additional Gyms that inhabit the Kanto Region. Maybe Ash could even earn some Kanto Gym badges for a change.
