I've read that the "Age of Wonders: Shadow Magic" is a game by itself, and not just an expansion. Is this true? Or do I still need "Age of Wonders 2" installed on my computer to be able to install and play "Shadow Magic"?

I've never known GOG to sell expansion packs seperately. It might be that people were saying shadow magic is LIKE an expansion in that it didn't add enough to justify being called a sequel. If it doesn't say anything about requiring 2 on the game page I think you'd have to assume its a standalone release.

Failing that, check the game forum for people complaining...

Quoth the news post:

"Age of Wonders: Shadow Magic is the final and arguably best part of the Age of Wonders trilogy. It is believed that it was originally designed as an expansion pack, but has matured into a complete, stand-alone game."

I wish I could play AoW:SM, but from what I read on this forum, AoW:SM have everything you need + more (like random map generator). You can also download/play scenarios from previous parts of the game.

In short: by buying/playing AoW:SM you will only miss AoW/AoW2 original campaigns/storyline.