The GoG version is the 1999 original one.

The Steam version features new scenes, new illustrations, new scenarios as well as a graphical revamp.

Just so you know. (I personally prefer the Steam version.)

I would much rather have the original version as an extra with the new version available as well. The new version is a mobile port and had a lot of stuff removed in order to make room on a touchscreen. I find the original better in many ways - little things like the the magic stone or manual saving, but they add up. Also the new version is far too buggy and crashes way too often.

The new version looks terrible, they destroyed the wholeness of the artistic design of the gui and use ugly fonts. And the gameplay is simplified. I tried it and went back to the "old" version - it's just a better experience in every way. The "remake" doesn't justice to this great and unique game.

It would be nice to be given the choice though about which version to play.

I was debating which version to buy myself and in the end opted for the original GOG version. The fact that Steam version only has autosave and is mobile port put me off right away. New illustrations and events would be nice to have, but not at the expense of game complexity and manual saving.

Pretty much this. They removed the different farm field types so now for the events that occur like drought you can't mitigate the drought by planting rye instead of wheat. They also removed the ability to set the amount of crafters you have (I think). Both of those are huge imo especially when I'm playing as a peaceful tribe because it's quite possible to never raid and be self sufficient on trade even when the weaponthanes get upset because you can just gift them with crafts. Now that seems harder or even perhaps impossible without the ability to assign the amount of crafters you have.

Edit: I originally loved the idea of the remake but its a shame they removed some stuff.

What are the resolution/fullscreen/windowed options of the GOG version?

In-game options? None. Basically, you can either run in it fullscreen at small resolution (the space surrounding the game screen will be black), or you can set it (via the compatibility tab in the properties menu of the shortcut or executable) to switch your display to 640x480 res when you run the game...which is fine, unless you need to multitask -- which you might well have to do, if only to reference the manual, which the game assumes you have read, or at least have at the ready. (Remember, it's a low-budget indie game from the late '90s made by a small team.)
That said, there are some things you can try to make your own options. =) For one example, see this topic.