Piracy is a copyright infringement. You are copying a game without having the right to. It's not stealing, which is taking something owned by another person without their consent. To use an analogy, it's rather like the following situation: you see a book lying around on your coworker's desk, you take it, make a copy, then put it back. Compare that to simply taking the book, carrying it home, and not putting it back. In the latter scenario, your co-worker will have lost a book, while in the first scenario, no-one directly loses anything, except for the copyright holder.
However, piracy is still a big problem. It's neither legal nor moral. People do it because they can get away with it, which shows you what kind of jackass many people actually turn into when there's no personal risk involved.
As for the general topic of the thread:
I don't personally know GDS, but from what I read on reviews, the games share common characteristics, but are by no means the same. That's not a big deal, it happens all the time, and it can actually improve the quality of the products in question. Take mobile OSs for example: when you see a great feature in a new version of iOS or Android, chances are a very similar thing will be implemented in the other within a short amount of time.
Not aimed at anyone in particular, more of a general statement: The only irony I see here is that people who download and play games, into which a lot of money and effort has been put, for free, claim the moral high ground when talking with the people who actually create the content. That and the high profitability of piracy websites.