nightcraw1er.488: If its a "GOG Key" then its highly likely to be fraud and the key will be revoked. Happens all the time. If I was you I would get a refund on G2A.
While G2A is known for not checking the keys sold there and it's possible to buy keys there that were bought using stolen cards, "highly likely" is a huge exaggeration. The majority of keys sold there are keys resold off sales or bundles, or bought off cheaply using regional prices, and never get revoked.
@detijdisrijp, the key is probably good, but if you can it might be a good idea to get a refund. People here are wary of G2A. It's a bit of a dodgy site and a grey area; I'm not sure if GOG allows reselling its keys.
Giving the key off here is fine, especially since you wrote where you got it from.