Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General GOG inDev and Divinity Original Sin 2

It was said during the Kickstarter that the beta might also be on GOG if they supported that by the time the D:OS 2 beta started, otherwise people could get Steam key for the beta and the full game on GOG.
There may still be details to work out, but it is certainly possible the D:OS alpha/beta could be on GOG.