I have no idea - I've had many keys from GMG and never had an issue with them, though I hear it can sometimes happen. Accepted your friend inv on steam :)

Thanks for your message on steam re: feedback. I hadn't left any yet as the initial key you sent didn't work.
I'm not asking or expecting that you should send another key btw - even legit stores will sometimes give out a faulty key by accident (at least, going by product reviews on some of the sites I've seen). These things do happen :)