The GAR model doesn't use Corsi, it rewards high danger over poor shots. I've included a lot of examples that don't use Corsi. Even the stats you chose, Gallagher has outperformed Granlund in 2 of them. I've provided you 'real production' examples too, players produce more than just on the scoreboard. Gallagher has been injured for much of the past few seasons, his pace still shows that he is last year is his norm.
Yes Granlund had more points than Gallagher. I could easily argue that based on quality of teammates, Gallagher could easily hit that mark in Granlund's role, Granlund had 20 secondary assists to Gallagher's 5. Not all points are created equally. So Granlund gets more TOI, gets better utilization, better linemates, etc etc etc. And yet the underlying numbers shows they produce pretty evenly. The underlying numbers show that Gallagher can be a 65pt guy if he were in Granlund's role.