To be honest I haven't either. Usually it has been *A everything plus 1K at the same time. When I boarded yesterday at EWR for a 6am flight, however, they specifically called *A Gold by itself. I was on the jetway before the next group was called.
I assume this was because the flight was heavy with status'd fliers.
It's even different to that usually (every UA flight I've taken):
1. F, 1K, Global Services - through red carpet line
(red carpet line roped off then)
2. Premier Executive and *Gold - through regular line
3. Boarding Group 1 (includes UA Premier, all *Silvers)
4. Group 2, 3, 4, etc
I've never seen them deviate from this.