I might be oversimplifying it a bit, but that's how it seems to me.
They both have the intended use of scattering the enemy team, but Gragas can do it from farther away, with less prep time, and dealing more damage, all on a lower cooldown too.
The difference is Zac's can take them in a specific direction altogether, while Gragas' is an AoE knock-out that would only do the same thing if you threw it behind the group.
Zac's ultimate also gives him immunity to CC during it.
Also if you decide not to channel and just immediately pop it again, it will still knock them up like his E does.
The difference is that Gragas' ulti is a lot less consistent in terms of where the enemy team ends up.
How many times have you seen even high level Gragas players knocking enemies to safety with an_ almost_ well timed cask, especially after the recent cast delay? Zac's new ulti (as far as i understand) will never do that.
It also looks like the fling distance is up to Zac, Gragas' cask knocks enemy the same distance regardless of what he wants.
Not to mention you can reliably hit Gragas ult, and Zacs ult is not even close to reliable.
Isn't it more comparable to a short Olaf ult? Immune to CC for up to 2.5 seconds, slowing enemies, and a 1000 range dash after.
Nah, it's more of a sidegrade to Graga's ult.
