Last batch of Gragas' nerfs seem to have hit him pretty hard. He can no longer heal without spending any mana like before since his W now costs 30 mana, and his waveclear is greatly limited. His W deals a good amount of damage and provides innate tankiness, and there are changes currently in the PBE to decrease its base damage - but that might be an overkill, in my opinion.
Ranged champions like Swain, Ryze and Vladimir can all deal with Gragas quite easily and outscale him into the late game. I really like going Ryze against Gragas because it's the matchup I'm most familiar with and have never lost. The only time he can kill you without you being able to do much is pre level 3, but afterwards you start winning every trade.