It happened so that I just won a giveaway for a $5,99 game of my choice from a very nice gogger who goes by the name of Grargar and now I can speak from experience. The best way to spend this amount is to put it in Gabriel Knight 1 + 2. After you've enjoyed them you'll get the 3rd one without hesitation :)

BLOOD!

Arcanum

I wouldn't recommend getting Star Control 1+2 here if you just want it for the second game because the second game is available free with the name: The Ur-Quan Masters. The website is http://sc2.sourceforge.net/

Not that old of a game, but if you like (very scary) horror games the Penumbra Collection is a very good deal at $1.99 when it's on its flash sale (it's on for about another hour right now). It also seems a good Let's Play type of game. I have the series on Steam, but it's very tempting to pick up a GOG copy at only two bucks. And I've only played the first game in the series... I need to go back and finish them all soon. :)

Anachronox.
Outcast.

Maybe keep hold of it in case they implement a way of using it in the sale at a later date? like the gog wallet/store credit people are talking about etc