Freespace 2 and Fallout 2 are the best selling games on GoG by far. Not really sure which one is ontop though but I would guess Fallout 2 is the absolute best seller, if only because of Fallout 3's recent release.
GoG is a great website and honestly... HONESTLY, if you can't produce $6... I mean, I hate to judge, but just the fact that you're posting on a computer and playing Freespace 2 means you're not living in a shanty town lacking all electricity and a source of clean water. It's important to financially support the developers of Freespace.
I haven't made use of the GoG service myself since I happily own the retail discs I purchased in 2000 or so but it's a great service nonetheless. Incidentally, a little known fact about GoG is that it's run by CDProject, the Polish gaming company that developed The Witcher, and they're a good group of guys to be sure.
free download freespace!... just kidding, well if your going to get it for free (and there are places that are secret squirrel) but the EULA can kiss my butt. period.
Hey now, none of that. This is a legitimate website, we'll have no such talk about those seedy 'places'!