...well, Gallagher can screen goalies...that play in pee wee...
Gallagher is in front of the net to fish for rebounds and thus to fracture the opposing defense. He's not there to screen the goalie.
He's there because Therrien doesn't trust the other options, e.g. Eller, Sekac, Weise, to provide that combination of getting rebounds and thus fracturing the offense as well as screening the goalie. Pacioretty is preferred at another position, Galchenyuk for the second unit.
...well, Gallagher can screen goalies...that play in pee wee...