I've owned WotN since it was released, now its unavailable to me. The icon in the main menu is grayed out, as if I don't own it, yet when I go to purchase another copy, the checkout tells me I already own WotN. I also had the cloud sync issue. I assumed by deleting config file, I could fix this issue as well. No luck. I've tried both the normal download and galaxy install. No luck. wtf is going on?

Sounds like something you'd have to take up with support:
https://www.gog.com/support/battle_brothers

Not sure if that's still the case but there was a chance of messing things up by installing in the wrong order. Basically any deviation from:

1. base game
2. base game supporter edition upgrade (optional)
3. beasts & exploration expansion
4. kraken banner upgrade (optional)
5. warriors of the north expansion
6. nordic banner upgrade (optional)

Remnants left in the registry from a previous, not properly (in reverse order) uninstalled installation could be the culprit too. Should Galaxy even check against something like that(?).
I highly recommend using the offline backup installers in any case.

Yeah, WotN still isn't showing up, but Blazing Deserts is. Crazy. I guess I'll ask support.

Same problem. Error with Galaxy cloud sync. Game reinstall solved the problem with sync, but now I have no the DLC WoTN. WTF. Support hello !?

If your DLCs are not enabled by default after install, go to Manage Installation/Configure from your Game screen or right clicking your game. From there you can enable/disable the DLCs you own.

I now how to enable/disable DLCs. But in the list there is no DLC Warriors of The North. However I 'got it in the library.

I that's a problem in Galaxy you should open a ticket/support in Mantis to advice about the problem.

I was able to solve the problem installing offline game distributives (last version 1.4.0.32) by hand, but after launching Galaxy app, the game was updated to version 1.4.0.35 automatically & DLC was missed again.

PS I've got ticket in support but I have no answer.

Sounds as there is a problem so contacting support is good. In the meantime it looks as though the "automatic update" can be toggled off. Looking at Redbat's 2nd post in this thread https://www.gog.com/forum/battle_brothers/blazing_desert_dlc_not_working

shows that auoto update can be turned off (untick the box).

perhaps
1) uninstall / reinstall base game (offline installers)
2) Untick Autoupdates
3) Install dlc's (offline dlc's)
4) play

I don't use Galaxy, but the screenshot in the post I mentioned above looks as though it might work.