Well this topic is quite old. Did they answer to your wish ?

I've just sent them a mail asking if they would consider adding Bitcoin payments in the near future, will post the reply when/if I receive it.

Great ! Let us know if you need any help along the process.

It would be helpful to add a comment to the petition if you already have an account there, or if you don't mind making one.
Here's the link to the petition again: https://www.gog.com/wishlist/site/the_ability_to_purchase_games_via_bitcoin

You can also send in suggestions, through their support, no account needed for that.
https://www.gog.com/support/contact