His passive used to be a flat heal on EVERY ability cast. Combine that with his free W cast and he's golden for lane sustain.
Q's damage got nerfed if you instantly pop it, which was Mage Gragas' primary form of poke. Now, it has to charge to deal the same amount of (or more) damage, meaning it's better for clearing minions who stand in the area, but not good against champs who can actively avoid it. It used to be an Attack Speed slow instead of a Movement Speed slow, so the added CC guides him towards being a tank more.
W used to give mana after a one second cast and give a buff after that gave Gragas AD and reduced all damage he took for 20 seconds, meaning it was his sustain and how he was able to last hit. Since it gave mana, it didn't COST mana, so it was free besides the cooldown, which was slightly longer than the duration.
Gragas' E used to be wider and have a bigger AP ratio, so it lost it's power with building AP.
Gragas' Ulti used to have a MASSIVE AP ratio, but it was nerfed alongside the base damage, making the ability almost all about the utility of the knockback rather than an alcohol nuke.
Basically, he used to be a mid-to-late game carry mage quite similar to Orianna, where he was safe in lane with mobility, had sustain to get through the poke from being melee, and had the damage to burst his lane opponent if his jungler ever ganked. Then he got nerfed a bit towards the mid of Season 3, then he was in this weird limbo of perma-banned but no good positions, then he got a pseudo-overhaul that made it better for him to build tanky in Season 4, resulting in the jungle/top tank you see today.