Can any attacks knock an enemy out of a Zarya ult?

Can you knock enemies out of an allied Zarya's ult?

Yes.

Abilities and attacks that can take enemies out of an allied Zarya's ult include things like Roadhog's hook, Reinhardt Charge. However, not all attacks can knock enemies out. Pharah's Concussive Blast, although strong enough on its own to knock players far away, does little to nothing in a Graviton Surge as it will pull the player right back before they can escape the ult. Same goes for Winston's ultimate -- despite how strong it is, playing as Winston I have never been able to knock enemies out of an ult -- as it will only deal damage.