ReynardFox: He probably wanted to charge too much, which is why braid isn't here iirc.
amok: It was more about rights, control and ownership, if I remember correctly.
edit - from the comments made at the time, Braid is not here because gOg was not that interested at the time, only in The Witness.
The game that isn't here because the developer wanted to charge more than what GOG wanted was Thomas Was Alone. Mike Bithell was quite vocal about picking the right choice and going without GOG, as the game did pretty good. That was when GOG's prices where more rigid.
Volume wasn't released here, either.
As for the Witness. Jonathan Blow was so quick to dismiss GOG after their offer that I always assumed that it was actually more about the failed Braid deal than anything else.