Hi, I'm from Mexico, then I can not access the game's page on Steam. I was wondering whether to purchase here I will receive a cd-key to activate on Steam??

I'm not sure why you would be unable to access the game's entry on the Steam store, but regardless, GoG is not a Steam reseller and you will not receive a Steam key if you buy it here. Instead, you will be able to download the game directly from GoG.

But you should get something like a Key so IF you get hold for Pirating Software, there should be evidence for saying : I got this Game legal, here is my Key or something like that :D Else you have a little Problem more ^^