I always look at the +/- compared to everyone on the team and relative to their 5v5 TOI. If someone is -20 and plays 10 minutes when everyone else on the team is at worst -5. That guy is a negative in his own zone.
That example doesn't apply to Gallagher, but in the first half last season, he was too often on the ice at the end of a hard game and out of gas. Lead to a goal against or stupid penalties. Marty ended up realizing it a some point thought. Gallagher just need to be used properly to what he can give now.