Hi, sorry for my bad English,https://support.steampowered.com/kb_article.php?ref=7480-WUSF-3601
Steam to the page you can buy it in another store and adapt it to steam.
I have purchased from GamersGate and inserted cdkey of steam is not valid ..
I noticed that my key is xxxx-xxxx-xxxx-xxxx while the steam is xxxxx-xxxxx-xxxxx.
I wanted to know if there are 2 distinct key, because I bought other games on other stores and placed in steam and had no problems.