You realize he can't deflect everywhere right? His double jump is also only effective if you're considerably bad at the game and can't aim. He has extremely low health with considerably lower damage than other offensive characters, making him only really effective against lower health supports like Mercy who can't fight back as well. A single Mei will essentially destroy a Gengi player, especially since she has nearly limitless survivability in comparison to him. Another good counter if you're bad is McCree, seeing as his stun is meant for fast moving and hard to pin down characters.