I thought GOG games were supposed to be DRM free? So why do I need serial numbers for the tropico 3 bundle?

I found the answer on the GOGWiki - http://gogwiki.com

"The original Tropico 3 with the Absolute Power expansion pack included, which features a new campaign and buildings. A serial key is provided, although using it is entirely optional as it simply unlocks the ability to submit your scores to the leaderboard as well as download challenges that other players have created, and create some yourself."

This sounds about normal. Although GOG games are DRM free, sometimes for multiplayer servers (or in this case sharing of scores and challenges) require a unique key to identify you as a unique player. GOG can control the game itself, but they cannot control the game servers as they don't own them.

Thanks for the heads up.

You can't even get to the main menu without entering a serial key.

Sure. It's a bug that need to fix it.