It's rather GOG who are selecting indie titles they would like to offer here - they'd like to get classic indie titles, or indie titles with the "old touch" (like Legend of Grimrock). Or something like Botanicula - a new game from a well-known Indie studio (if you can say about indie studio that it is well known).
Sending those emails to indie devs you are making them (those who are interested) thinking that it's the option for them/their games.
And in some cases it's even true, but I think that for most devs, GOG won't be interested in their games.
Don't forget, GOG is not going to release a pack of indie games at once. And since they are limited to two (or now maybe three) releases per week, if they would consider getting all those indie titles here, they'd have to stop releasing other games (oldies and newer). That's not going to happen, so the answer is - most of those devs never going to appear here. Even if they would really like to.