They do appear to be "kosher", as in you get genuine full games with no malware attached, but there is a caviat (see below) .

These games are rarely(?) DRM-free (the activation tells you that, but some people misuse the term - I don't know which term is appropriate, if any, for this particular game), but may or may not be portable after installation (if you're lucky AND you know how to back up the right files, the game may or may not work when you restore it at a later date - otherwise the game will continue to work so long as it remains installed on the same computer - again, because of the DRM-protected installer).

Many people don't care about being able to install it later, but some do, so you should be aware of this. The site's business model seems to be to give you some limited-lifetime games in the hope that you'll come back for more (and eventually pay for a game you want to keep); if you always install each free game immediately and never need to install it again, there's no problem for you - it will continue to work so long as you don't need to re-install it for any reason (e.g. computer upgrade or operating system recovery).

EDIT: by the time you like a game enough to want to re-install it, you may not mind paying for it anyway; this would probably apply to me but the for fact that I'm unlikely to actually get to try the game at all before I have to re-install it.

In fact, the original downloaded installer is usually useless the day after the giveaway (because of the DRM/activation), so it just wastes space to keep it. Again, you need to know how to back up the game files and know whether those will actually still work when restored.

You don't need special knowledge to back-up this and other portable games. Just copy and paste the folder of this game.
Not every game of GGOTD is portable, though. But this is.
It's mostly mentioned in the review.

The copying part is trivial for you and I, perhaps, but not necessarily so for the casual gamer. And even I have difficulty telling whether the game will work again or not afterwards.

It's handy to know to look for this type of information in the review, thanks.

Thanks for the info. I think I'll just pass, then.

Giveaway of the Day - Kronville: Stolen Dreams

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GabeN: The Final Decision (50,000 Keys)
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Indie Game Battle (30,000 Keys)
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Giveaway of the Day - Tales of the Orient

It is interesting to note that this game comes in a self-extracting archive; meaning that the game is DRM-free and it can be rezipped again and backed up for future usage (on a hard drive, on a cloud service, your mileage may vary).

Thanks for linking to this, I was planning to get the game for a while now.

Have fun with the game :)
I'm going to install it now, I like some match 3 games now and then.

Most or all of the Alawar and Immanita games on GGOTD are like this.
I stopped mentioning that fact, because there are a few users who just can't believe that. (Scroll up)

Playrix games need a full install with a few files "hidden". They are not portable without finding those files.

The self-extracting archive is wrapped in DRM, and therefore is itself not recoverable for later use. However, in this case it's easier to determine that the game is likely to be portable if you re-pack it after installation.

Knowing that the game is easily portable is useful information, even if we disagree about the DRM.