Voted, but I'm sure it has been announced somewhere at the Thimbleweed Park blog that the game will be available at GOG. :)
I actually got an answer from Ron personally there.
Okay, I must admit, it wasn't the answer to my question strictly speaking. My question was "can backers get GOG keys" and his answer was "We will be on GOG, but I don't know if they will give out keys to give to backers". https://blog.thimbleweedpark.com/pledge_faq
(search for "Vainamoinen")
Glass half full. :)
/edit: JUST EIGHTY-FIVE VOTES WTF IS WRONG WITH YOU PEOPLE
Well, people aware of the game may already know that it will be released on GOG and also consider that there's little point in voting for something that will happen anyway.