Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General GOG vs Steam

YES, you can get Alpha/Beta access through GoG. But It will have to wait until October.

Yes it's a mild delay and that sucks, but it's not that big of a delay. The Alpha will probably last for a minimum of 3 months, so there will still be plenty of time for your feedback to make a difference.