As we all know, Steam has a "betas" feature that Paradox has exploited to let the player download and play old versions (of CK2, EU4, etc.), rather than being forced to play on the latest patch at all times. This is a good thing because (1) the newest version may not have had all its bugs ironed out, (2) the newest version may have introduced features that the player dislikes, and (3) mods that the player wants to use may not have been updated for the newest version.

However, when I look at the download page for my GOG copy of Darkest Hour, I see that only the latest patch (1.04) is available, with no hint of any older versions. I don't have the Steam version of Darkest Hour, but the Paradox forum tells me that on Steam the 1.03 patch is still available under the "betas" tab. Am I correct in assuming that the GOG release of Imperator will likewise not make old patches playable?

When I posted this question on the Paradox forum, a Paradox employee replied: "I think only GOG could answer this question."

You can download an offline installer, of any game, for every version released, and keep all of them.
I don't expect GOG store will have those available, and will only have the latest version.
Especially when multiplayer, modes are involved.

Also unlike Steam, GOG never forces you to update a game.
Even with Galaxy, you can choose whether to use auto update for each game, individually,
So there's no real need to use a Beta channel on GOG to roll back to the last update.

The user can always decide when, and if to update, on GOG.
Just turn Auto updates off, for the game, if using Galaxy, and simply don't install it, if using offline installers.
You can save all older versions, and/or patches as offline installers, if you want to.

Thats not completely true. When you click on a specific installed game in your galaxy library -> more -> settings -> and uncheck "update to the newest version automatically" then you can see different versions of the game. I don't know as many, but I think five patch-versions ot the game.