How?
Players voluntarily go to head the ball. Gabriel getting 'hit' by the ball is just not violent conduct!
Not even close to being violent.
Unsportsmanlike, 100% yes. But violent? Nowhere near. If you're gonna hand out red cards for that, retrospectively or not, that to me negates what actual violent conduct is (biting, intentional elbows to the face, punching an opponent, studs high up intentionally etc).
If that's classed as violent conduct, then so is what Fuchs did to Sanchez (even though the latter deserved it for his fannying around at a throw in). And that wasn't violent conduct at the time either.