I can’t say how easy or hard it would be to pass through their curation process, but I think 0 A.D. would be a great fit for GOG, if distributed there as a free game. It would no even need an "early access" label (it is already in a much better state than many *released* commercial games).
Many GOG customers are starved for RTS games, and multiplayer games that can be played without going through a store launcher/DRM. Many of them joined the store for "old" games and are still waiting for Age of Empires, so having access to 0 A.D. could scratch their itch.
I expect launching the game on GOG would have a bigger effect on 0 A.D. popularity than what itch.io did, if only because of the small catalogue leading to each game getting more (relative) exposure.
To get an idea, here is their full list of so-called real-time strategy games (most of these are not even what we would call RTS): https://www.gog.com/en/games?genres=strategy&tags=real-time&hideDLCs=true
It would be very easy for 0 A.D. to shine when compared to most of these games.
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On the topic of the alpha version, I too am in the opinion that it does no longer help. I would suggest dropping alpha, beta or whatever and stick to the 0.0.27 naming scheme (0.1.0 would be first beta, 1.0.0 first "complete" release). Alpha/beta could stay in internal discussions, but no longer has an useful meaning for distribution.