I've seen a link to this site on "chip.de" about 10 Minutes ago:
http://www.fileplanet.com/promotions/groundcontrol/
Here you can get GC 1 for free the legal way.

It's the first game only, doesn't have the expansion pack, Dark Conspiracy. You could argue that you buy the expansion at GOG and get the first game with it for free.

[url][url=http://www.fileplanet.com/156136/150000/fileinfo/Ground-Control-(Free-Game)[/url]http://www.fileplanet.com/156136/150000/fileinfo/Ground-Control-(Free-Game)[/url[/url]]
Here it is without the sign-up thing (that is, as long as you have an IGN/D2D/FP account already)

I actually first got into Ground Control with the free Fileplanet version, and I really wanted to play Dark Conspiracy, so I got the GOG version. Great game, though the later missions are getting pretty hairy...