By the sounds of it, GOG is getting the alpha/beta but it will be delayed compared to the Steam Early Access version until October (Like ~2-6 weeks later I suppose?), however the full release of D:OS2 isn't until sometime next year so you'll still have the alpha/beta for most of it.
Divinity: Original Sin II is planned to join the Games in Development program on GOG.com in October 2016.
I assume that's GOG's new early access like program.
