I can't seem to get any achievements, and the funny part is I see my friends who played the game years ago have some of them unlocked...
I've searched the forum, and I've even googled for it, but with no avail.

PS it was the same story with Monkey Island 1 SE :(

Recently completed them, no achievements on gog.
I believe, these are implemented on steam, and maybe you just have steam integration turned on and misinterpreted that as being unlocked on gog platform.

Yep, that's exactly the case. I didn't notice the tiny steam icon on the right.
Case closed! :)

That's to be expected even on Steam.

I have all of them on Steam and only Monkey Island 2 has achievements. GOG store page for Monkey Island 2 does not show achievements, so I guess they have not been implemented on GOG.

But the achievements are mainly just from playing the game normally and a couple of weird ones that only pedantic people will attempt. You're not missing much if you don't have achievements for Monkey Island 2. Just enjoy the game.