Muskyhopeful,
Assuming Parthenon is the first in the US. Let's say it's the most authentic (American version)
From the pics and forum:
The gyros looks like it's made with a blend of minced meat (25% beef and 75% lamb)
It also looks like the most popular representation of what is identified here as Gyros.
This is a menu of a well known Greek fast food chain in Montreal. (Gyros is spelled yero in their menu)
Kojax Souflaki Restaurants
Luc H