Can anyone verify whether the GOG version of FA is 3599 or 3603?

The 3603 beta patcher can't recognize it, either. Replays from my Steam version are listed in the replays menu but can't be played.

I sent them a support ticket with a link to the 3603 beta patcher, let's see what they say.

Great thank you, I`ve wrote on this issue also in the release thread. 3603 is especially important, as it adresses multiplayer balancing.

Have you recieved an answer to your ticket?

Nothing yet, I'm afraid. Nobody has yet been assigned to it. Considering the sale and work-from-home policies, we shouldn't expect a hasty response.

I think it's best to have the game file at version 3599 because the latest official version is at 3599.
However unreleased beta patch 3603 from the original developer should be available for those who need it.

It was demonstrably released. It's what the Steam version is based on.

I actually wonder what benefits it brings? Is there a changelog for it?

Its mostly about balancing, which is especially important in multiplayer (of course also skirmish). I have no changelog (read it years ago when the patch came out), but as far I remember e.g. Experimental Units have been balanced.

I understand if this patch could also provide for us some servers to play over internet. Even basic P2P would've been something. Skirmish balance isn't that important if there is no changes for AI, as it's overproduce land t1 and only use around 1/3 of it even as cheating attacker. They can use artillery and nukes, but there is no challenge from their armies.

http://forgedalliance.obliteratingwave.co.uk/Patch_Info/1.5.3603.html

Thanks.