I know this isn't exactly a old game, infact it's brand new. But it's a very good game, and by an independent developer/publisher that seems to advocate DRM-free. I'd much rather purchase it through GoG given the chance, than through a 3rd party such as Steam or such.
If by some small miracle someone is unaware of this, it's made by 2dboy.com all information is on the site.

Exactly why not.

I thought World of Goo was only on the Wii.... ?

World of Goo is coming to WiiWare, Steam, and retail stores on October 13. I pre-ordered the game, so I'm playing it right now. :)
Also, there's an option to buy directly from him, which comes from a download, I think.

They have a no DRM policy.. so if you buy direct from their site (via paypal) then its DRM free.
AFAIK the Wiiware version has multiplayer... the PC version doesn't. Don't know if there are any other differences..
Not sure why it needs to go onto GoG when you can buy it direct from the developers. Go support the developers. Or get it on wiiware and hope that it sells well enough to encourage some more awesome games on that platform.

Nice! Depending on how much it costs, I'll very likely buy it. Looks really fun.

It's 20$, not sure how that fits into GOG's scheme, if at all. Publicity would be the major reason 2dboy would distribute the game through a third party - so that depends on how many extra sales that'd mean. To the wishlist?

World of Goo is a really lovely game, but it's Brand New, not Old :) (not that I'd mind seeing new games here, but it would mean cutting the price in half if the $10 maximum was adhered to, so that's awfully unlikely at this stage).
Just buy it direct from 2D Boy's web site. No DRM, and the money goes directly to the developers.

Also, doesn't the game involve condoms?