So, i preordered retail copy that comes (like all of them) with GoG code. Store tells me, it will be available on 10th of dec. Can i, in someway, preload game early and activate it, once I get my retail copy?

I doubt it.
If you don't have a code/own it on a digital platform, how do you tell gog you legally own the game?

No.

You need to add the code to your account before anything can be downloaded. So you won't be pre-loading it like everyone else did that did pre-purchase it through the GOG site/launcher.