Well, for starter, I don't know what the first sentence is supposed to mean. Secondly, you threw in Panik into that mix. That pretty much shows you know absolutely nothing.
Habs fans do tend to overrate players, but that's not the case with Gallagher at all.
Want to go with "points"? 12th best in total 5v5 points amongst RW in the past 3 seasons. Despite significantly less ice time than his peers due to the Habs rolling a 2-lines system. Remains top 30 when you factor in powerplay, which is indeed not his strength. But the Habs PP strategy is also incompatible with how Gally plays (and coincidently, our PP is bad but we still try to feed the puck to Weber for slappers, figure). On a team with a good PP strategy, he would totally have 5-10 more points easily.
Goals? He's one of the best goalscoring RW in the entire league, no matter how you put it. Not by skill, obviously, but by how he plays. And how he scores matters only for entertainment. A goal is a goal whether it's the result of playing whack-a-puck in front of the goalie, or the result of a shot that results in the goalie being dehydrated because his water bottle was sent in the stands. Nobody in the entire league shots more than Gally does, and that's part of why he's more impactful than his point total would state.