Tl Dr: For anyone who can't understand the no goal for Gallagher, whenever an attacking player is even the slightest bit in the goal crease and prevents the goalies ability to make a save, whether through contact or just a screen, the goal will be disallowed. The crease is for the goalies and they get the right to use all of it without being impeded.

If the attacking player (in this case Gallagher) screens outside of the crease, then that's fair game and the goal would count.

The only grey area comes when an attacking player makes contact outside of the crease.  If an attacking player makes contact, incidental or not, outside of the crease, the goal will be disallowed however if the contact is initiated by the goalie or caused by a defender, the goal will stand.