The gym's design is structured with an ice slide leading down to the battle area. There is no way to climb back up, and the gym leader, Brycen, is waiting for you at the bottom.
It's possible you're thinking of a different gym, or maybe you're trying to access a location that isn't intended to be accessed in the game.
Let me know if you're trying to accomplish something else in the gym. I'm happy to help! 😊