No. But you're closer.

What's the genre? RPG?

"Turn based strategy"

The quotation marks are because due to a design flaw it plays more like an RPG.

Is it on GOG?

Yes. A pretty famous series. Though this game in particular isn't the most popular one.

The game sold on gog is the gold/diamond/deluxe/director's cut/goty version, that includes all expansion packs by default.

A wild guess... Fantasy General?

No.
It's an extremely popular series. The game in question though was criticised by many fans (though it does have its fans too, and personally I didn't find it all that bad overall, though it did have its flaws), partly because of the fact that it plays less like a strategy and more like and RPG.

The setting is medieval-fantasy-magic type of setting with your usual humans, elves, dwarves, orcs... etc... etc...

another wild guess here: might and magic heroes 7?

Very close, but no.

OK, I feel stupid. I got fixated on the name "witch". But I've just got a revelation... you actually Evil Sorceress from Heroes of Might and Magic IV: The Gathering Storm.

Yes, of course.

EDIT: I initially purposely used the word "witch" to make it more confusing and google proof, then forgot about it.

It's a bit weird... I have to visit a huge spider because only there I can get more healthy. I really need it because I must leave my body to become more powerful. But to get this power I have to be in a pretty good shape before leaving my body so here I come spider!

Wow, I don't remember that enemy despite playing most of the campaigns in the base game and some in the expansions. I should probably try replaying it and finally finishing the story with all of the expansions! :)
Is it a scifi by any chance? The first body hopping game that comes to mind is Omikron, but I don't think you get to jump to non-human bodies in that game.

It's not Omikron.

And I didn't mean that I had to become a spider. I only mentioned that I had to visit him to get healthier. And then I left my body...