I have this game on steam (the special edition version), and I can tell you its quite excellent. The soundtrack is incredible.
One difference between GOG and Steam's release is that vanilla and special both get Shadowgate CLassic COmic with GOG. Steam didn't get that extra. Although that comic can be found on the web for free. Either through ZoJoi's website, or wayback machine on Infinite Ventures website.
Also keep in mind that there are three modes.... three difficulties, and each one changes puzzles, adds new ones, etc. So its almost like having three games in one.
Plus there are literally dozens of secret puzzles for unlocking extra items and achievements, extra plot, etc. SO there is quite a bit of replayability. Some of the deaths are well hidden, and require their own puzzle solving to even find them! Finding them leads to achievements.
If a company is going to make a remake, this is actually probably the model they should use! More features, not less!
Achievements are worthless. Steam achievements are just 'there' to make you think you're getting some value out of your game that you are not. It takes zero effort to do. Also it doesn't matter, you don't get achievements on a DRM free, launcher free version.
I have my own beef with the achievement system that has been the standard for over ten years. They add nothing to the game, but steam (Or whomever, even game devs themselves) want you to 'think' you're getting some incredible value because 'wow look, we've added 50+ achievements! ZOMGWTF'. Sorry that would never make me do anything other than the 'jerking off' motion with a stupid face and then ask 'okay so where's the real content?'