I think the problem you will find is that there is not one single "traditional gyro". It's a wild goose chase.
Instead there are variations on what gyro's mean across the world. In one area they mean one thing, and in another area something different. In each area you can say "that is an authentic gyro", but that same gyro won't fly somewhere else.
So, I think you can take ANY definition of Gyro, and replace the "ground meat" part of your title with that, and find problems with the definition.
It's like asking "When did BBQ become anything other than pork"?