I bought this game a long time ago and thought I'd start it up again today. If I don't play a game for a long time I end up uninstalling it, so I had to reinstall it.

I was thinking, ok, GoG, no DRM, no problem... except it says that I have to register in order to play in other than demo mode. And then I get the "too many activations on different computers" thing, despite this being the only computer I've used for it. Isn't limited activations a form of DRM?

I just sent an email, and just got back what seems to be an automated response, basically talking about regions of the game and setting time zones appropriately... which isn't my problem. I need to be able to re-register it. So I sent a message to "support@topware.com" and hoping they will be able to do something. Just wondering if anyone else has run into this and what the outcome was.

If you have the GOG release of the game then you should be contacting GOG support, not Topware support. Topware would be fine in supporting the original release, but the GOG release is modified to strip out DRM and to help it run more easily on modern systems.

The DRM is removed from the GOG version as in you don't have to activate in order to play it. There is an activation button present (but I've never had to use it, but have only played single player). I had to request a unique CD key from GOG for multiplayer, though I've never actually used multiplayer.

Yeah, apparently that's the way it works out. I was laboring under the misunderstanding that I had to register/activate it or it would only play in demo mode, but that's just for multi-player, it seems.

I'm afraid I'm finding that bogus. My GOG copy of Earth 2160 will do NOTHING without this key!
Even the editor will not run without it!
And I'm not happy with GOG's breach of their "yours to keep forever no DRMs" promises on this one!

That post I made was from over 2 and a half years ago!

But anyway, myself and other users have been able to play without using the key, so it is not 'bogus'. It may be something to do with software you have (or have had) installed. For instance, if you had the original retail version installed at any point then it may have left behind remnants in registry or similar that make it want to keep looking for a key.

Regardless, the advice is the same as I posted all that time ago. Contact GOG support about the issue and they'll try to help you solve it.