Is this ticket response legitimate?
If they sent it back to you in a ticket (not email), then yes it is legitimate. I had to go through the same process to link my GW1 and 2 accounts together and they sent me the same thing. They can’t do much with the last 4 digits of your CC, and having your used serial number won’t do anything but prove you have used it already for your account. Any new user trying to use your serial code would get an “invalid serial code” error. In short, yes it’s legitimate, and I hope you still have all that stuff.