I've reached out to them regarding a "coming soon" page, but our current demo (still in development) won't be available for purchase via GOG because it's free! So Legrand Legacy will only be available in GOG for the full version.
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Demos are treated differently on GOG, they have separate page entries from full games; I could find you a few examples if you need.
Games in development listed on GOG can be pre-purchased, basically (at least most of them, but I remember some with prices marked TBA, therefore you could not purchase those).