Should we mark as "The product obtained free"?

No.

Yes. It doesn't say who has given free. Additionally many users do give promotional keys directly from developers.

That option is there for the reviewers that got a free copy straight from the dev. Think of youtubers and such. Nothing is stopping you from doing the same but i dont feel its necessary.

That checkmark could be loosely translated as "Got paid for this review".

I say no due to the insinuation that one would draw from your review. Peeps will assume that you contacted the dev directly and not that you were gifted the game from some random on the internet.

That option is more about stressing you had personal contact with game developer/publisher, which could seriously affect how you see this product.

So, most of times there's no need to click it when you review game you won on SG. Unless you're reviewing one of Developer Giveaways.

No because even though you got it for free, someone else paid for it. Meaning that the key itself wasn't obtained for free. Only mark that if the game was actually free or if you were provided a copy by a developer

That checkbox is meant for products received for free for review purposes. So you don't need to check it for games won on Steamgifts.

No, that's meant for review purposes, as if the developer has given you the game for free.

Of course, if you win a game here (usually one of those big 50+ copies ones) that's from a developer for promotional purposes, check the box.

someone paid for it, it wasn't donated by the developer.
well that actually depends on who made the giveaway since the new trend is to give away dev keys and farm levels with them... so generous!

so yes, I would mark all my wins from x10+ copies giveaways as free.

Thanks for answers. Have another qustion - if I trade game or get it as gift from other Steam user - still no mark?

It's your decision, but the option is there for people who get free copies for review. For me, this might apply to a few mass GAs made by developers here, but not for the rest.