As in the subject, after updating from 1.0.4 the tab for selecting custom/community scenarios is no longer there.
I've been trying to use both galaxy and stand-alone installers but it didn't help.

I can't find it either. It was a third tab after Freeplay and Challenges.

So, nobody knows what's the problem so far?
Any devs out here to address the issue?

Maybe it's because of not having the Steam version. Isn't the Community tab part of the Steam install?

Well, community should not be limited to Steam Workshop. What about other creators who want to create their own scenarios and distribute them outside the workshop?
The tab for community scenarios was there in v.1.0.4, but, apparently, was removed in later updates for a reason uknown to me. If it's the case that GOG version will not support custom content anymore then I want a clear statement on this because it's no longer the product I purchased.

It's still possible to add custom content, I wasn't even aware of the community tab in the game. Did it work? I can't image unless the devs have their own platform to host it.

The custom content I know of is available on different mod sites like Nexus: https://www.nexusmods.com/dawnofman

But how do you play custom scenarios?
Community tab worked in a way that if you put the scenario files in ../Documents/DawnOfMan/Scenarios/YourScenarioName/ it appeared in that tab and you were able to choose and play custom scenarios from there. From update 1.0.5 there is no item in the game menu from which I could start my own scenarios.

I can download it and add it, but can't play it.

I agree. This is a question that deserves an official response.

It's not even answered on the Steam Forum

- https://steamcommunity.com/app/858810/discussions/0/1812044473319824012/ -

Recent answer from the devs in the topic posted on Steam:
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We've checked this, it looks like we've accidentally removed the Community tab on the GOG version in 1.0.5 (when implementing GOG achievements).

We will restore this for 1.0.7

Sorry about that.
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Ah that makes sense. Great to have an answer. :D