I too have the same issue. I just noticed it a few days ago and have not figured out a way to change it. I was able to add the certificate of the v9 core to the gateway but thats as far as I have gotten.
Just an FYI if you have not found an answer to this. I contacted support and they help me find the old 8.8 cert and remove it. Once i did this i was able to add teh new server to the gaeway