You need to wait:

Quote
WARNING: Do NOT get a Steam key now if you want to play this game on GOG. Keys are currently only available for Steam, but keys for GOG will come later. You will NOT be able to swap a Steam key for a GOG key. If you want to play on GOG, you will have to wait until Divinity: Original Sin 2 is available on GOG.

https://www.kickstarter.com/projects/larianstudios/divinity-original-sin-2/posts/1667422