G2A.com allows sellers, such as Plyxa_games, to mislead buyers in that they (the buyer) will receive a Steam Activation Code.
However, the code is not revealed, and is rather sent to a dummy account that Plyxa_games creates (unrelated to your actual Steam Account. The game is then added to that account which is only useful if you sign out of your Steam account and into a new one. You will not be able to attach the game to your existing Steam Account (The reason a person would want a "Steam Global Key" in the first place).
I cannot say this is 100% scam, but more of a buyer beware.
I have contacted G2A on multiple attempts, only for them to respond that: "I believe that the case has been clarified and explained in detail so the ticket will be completed according to our procedures."
The above was a reply from a G2A rep, Gabriel D.
In other words, Gabriel (G2A) doesn't care what the customer has to say and he believes it's closed, so, end of story. Not a great way to do business.