Hyper Beam does a lot of damage on its own, sure. But again, Chansey has horrific stats. It has practically nonexistent attack stats (read: LESS than a Magikarp's). And just because it has a ton of HP doesn't mean much. It has no defense stats to go with it. I doubt you're going to fire out three Hyper Beams unless you're fighting sub-CP300 Caterpies and Weedles or equivalent Pokémon with abysmal stats.
Chansey in Pokémon GO behaves similarly to Chansey in the core series games. Just poke it once with any physical move and it deflates like a balloon, even with perfect stats. And because there's no discrepency between physical and special attacks in Pokémon GO, Chansey will melt to practically anything remotely competitive in the game.
Obviously you can use whatever Pokémon you want in a gym, but if you actually want to make any sort of impact, a Chansey isn't going to get you anywhere. ESPECIALLY on offense. Chansey is a trophy Pokémon known for its rarity, not its combat potential. Maybe when Blissey becomes available we can revisit this matter.
P.S. I guess if your goal is solely for prestige leveling, you can use a Chansey to solo a horrible ~CP1000 Pokémon for fast experience. There's only value in this if you have a coordinated team with the ability to fill the gym up fast with a proper lineup.