Hey,
Thank you for your contribution and sorry for the late reply.

There will NOT be a GOG release.
Quote from the GOG answer from 2023:

Thanks a lot for your submission and your interest in GOG.
We’ve taken a look at Summer Islands, it looks like a managerial game, with fun mechanics and simple, yet eye-catching graphics.
Unfortunately, however, we feel that the game would not be a good fit for GOG, as we think that it appears to be too niche and a bit too small in scale in terms of production value for our users, which means that we aren’t confident in its release potential on our site.
For these reasons I’m afraid we will have to pass on Summer Islands.

The Steam Workshop will currently only contain the maps from the Map Editor.
Other things such as buildings are not currently planned.

best regards
Matt

the people from gog are crazy... they have "have a N.I.C.E. day" and "N.I.C.E. 2" now, and this is to "niche"? i.cant.even.

owell thanks for the reply

those that want to see it on gog let them know https://www.gog.com/wishlist/games/summer_islands

Sad to read such a response from them!
(Especially, as ¥‎€‎‎$ wrote it, when there are many niche games on GOG...)