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It has just been released on Steam. I'd rather buy it here, but it's so hard to know if a game will ever be published on GoG (the amount of games I buy on Steam because they're not here and then "puff", it becomes available, is staggering).
As a big Agarest fan, I'd also like to see it here on GOG.
There was a thread on the main page. In summary: extremely unlikely.