nasKoo: Currently, the work required to upload and maintain IWBUMS builds on GOG is too much of a hassle in comparison to Steam (at least to the best of my knowledge; if that has changed, I'd be happy as it would give me something to convince the devs with).
Multiple updates over the course of a few days would often hang in limbo for additional days due to builds requiring GOG approval first. That would mean we wouldn't be able to get the feedback we'd need in order to fix bugs in a timely manner.
While the userbase itself is obviously a fraction of Steam, the percentage of said userbase interested in IWBUMS seems to be even smaller, so I am not entirely confident that I can successfully rally for IWBUMS builds on GOG.
From what I've heard, it is possible to directly push updates, without GoG approval, by using the beta channels feature of the Galaxy client. (But for more accurate information, better ask GoG).