Right now Crawl is in early access. And if I'm correct, GOG does not release early access games. (Please correct me if I am wrong!)
Here is a quote from Steam, which in turn should be a quote from the developers:
“We have enough planned for Crawl Early Access to take us through to at least Early 2015. Beyond then we'll see if we're still having new ideas for things to add!”
So my guess is 'at least Early 2015' :P
This is a shame, as I'm really looking forward to this game... it looks awesome. I have made it a point to focus my game purchases on GOG, but seeing as this game has been available for a while and the GOG release date is unknown (possibly next year) I'm not sure what to do.