Since Byrne was a guest penciller on issues #164-6, and Gyrich was a character Shooter clearly had plans for, it's quite possible that Perez and Shooter designed Gyrich, and that Byrne pencilled his first appearance in #166 from Perez's design.

It would hardly be the first time a character was designed/co-created by one penciller, but introduced by another due to production problems or schedule shifts. I suppose someone will have to ask Byrne at his board. Not it!