Instructions
Purchase several pokeballs. You can find them at stores throughout the game world. Since your odds of catching the Gligar will never be 100 percent, you should bring at least a dozen pokeballs with you.
Travel to Route 45. It's south of Blackthorn City and east of Route 46.
Wander around in the grass. You'll randomly encounter Pokemon, and on any given encounter, there's a 20-percent chance of the Pokemon being a Gligar.
Attack the Gligar to weaken it. Get its health meter to turn yellow or red.
Throw a pokeball at it. If you're successful, you'll catch the Gligar. If not, throw more pokeballs until you do catch it.