I have deleted e a full malware and virus scan of my server (automatically done every week) and there is nothing to indicate the origin of these popups is GMG.
I suspect it is some form of malware on your personal machine. From your signature, I can see you are Windows-based and fairly cluey with regards to IT stuff. When you get the popup, try a different browser or even a different machine (laptop, tablet etc).if it only appears on one machine/browser combo, then we have a suspect.
Please keep me informed.
If anyone else is seeing the same thing, please report here.