I am guessing the window seat the gate agent unblocked is next to the pilot rest seat. It's supposed to be the last seat assigned to a passenger, since they are supposed to assign all other seats possible before putting someone next to the rest seat.
UA, in general, does not block premium cabin seats outside of ones reserved for pilot rest. Sounds like you booked fairly last minute, and there were no other seats available other than the middles. This is not due to the fact that you're *G - same rule would have applied to others as well, including UA elites.