Originally Posted by GeneralDirection
The thing is all of Gragas's spells deal magic damage. By doing physical damage, you're abandoning nearly every ratio that supports him. Many of the successful champions on Dominion play to their scaling abilities, and if Gragras doesn't do the same, he's just not getting much value per level up.
W gives him 70 bonus AD
, and 18% damage reduction (not armour, not MR, pure damage reduction). This is one of the biggest AD steroids in the game, more than Trynd and Sion, behind Mundo and Malphite.
E has a .66 AD ratio. Plus a 35% slow.
Q has a 20-40% AS debuff.
His ult displaces, which also interrupts channels.
You're right that his abilities deal magic damage, so there's that awkwardness of building arpen for your autoattacks and it not benefitting your abilities. This lack of synergy is probably what holds him back from being on the level of Jax/WW/whoever else. But Gragas is actually set up as a really decent bruiser.
Triforce is an easy choice on him. He scales with every single stat it provides, and his spammable abilities go well with Sheen.
Grab a Frozen Heart for the armour, mana, and CDR, and with your barrels, you can shut down enemy AS by 60%.
His #1 weakness is having to channel W for 1s every 20s.