I have emailed 5 developers regarding missing content / updates. the games in question:
Tower of Time - Linux version outdated + No FULL OST (available free on Steam), latest ver also adds French language.
SteamWorld Heist - Devs said in an email it would land late Sept 2018.
Victor Vran - Linux version missing 1 patch, iirc GOG encountered issues with this patch and rejected it, I asked about an ETA.
Hyper Light Drifter - Mac/Linux are both updated, the last few patches are only available to WIndows users.
The Signal From Tolva - Linux version missing 1 patch which also happens to fix a few important issues.
I shall keep you posted should I get any response / updates. Status Update (all responses were sent to my personal email):
The Signal From Tolva - Devs responded that they sent the latest version to GOG back in August, but somehow it didn't go live. They also mentioned that pushing Linux builds is done manually (probably due to lack of Galaxy).
SteamWorld Heist - Devs responded that they sent the update to GOG.
Tower of Time - Devs responded that they plan a big update coming very soon, along with the full OST.
All in all, things are looking good so far. Kudos to these devs that responded in a very efficient and professional manner.
Further updates on these? Want to be sure which I do/don't add to the list.