yeeiser: A friend of mine had to buy it through G2A and gifted the key to his brother. You can also buy it on Steam which comes with a GOG key and you can gift it to your significant other
blotunga: I wouldn't recommend buying it through G2A. There were lots of instances of revoked keys because they were bought with stolen credit cards and resold on G2A. GMG sells GOG keys afaik, and the Steam version doesn't yields a GOG key (only for the first two games it does).
G2A has something called "G2A Shield" which is just an insurance that confirms that the key is 100% legit, it only costs like 1.20$ for every purchase (and its optional). I have never used it and there was only 1 time when I had a problem with a key, I sent a mail to G2A and 2 hours later I got a refund
Sure its a grey site but its not that bad