Slight pushing in the box is part of the game. If a player falls down every time an opponent challenges for an aerial ball, you aren't going to call a foul each time. Gabriel went down weakly from an awkward position he put himself in.
To me, Gabriel was intentionally dropping down to flick the ball backwards, not pushed down. The added pressure from Bruno made his header trickier (which is literally what challenging in the air is supposed to do) but it was his odd position, and not that Bruno's pressure was more than allowable, that made him miss.
Then, because he saw it was going wrong, Gabriel collapsed so to give the ref a reason to deem it a foul. Like so many defenders do these days.
They also control their games and aren't doing quite as many full field sprints to track back. Or, at the very least, that's what I suspect the stats will back up.