if you're talking about the gyms in sinnoh, we've had
1, rock type
staravia is at a direct disadvantage and he didn't have buizel or gligar/gliscor, so they weren't used
2, grass type
staravia was used. didn't have buizel or gligar yet.
3, fighting type
staravia was used - took out 2 pogleys. buizel was also used. gligar wasn't used - it was a fairly new pokemon and i dont think it had even had a battle yet.
4, water type
as a water gym, buizel had the advantage of being able to swim. used pikachu and turtwig since they had advantages. gliscor at a disadvantage, staravia didn't have an advantage.
SO you see, its just common sense, really. buizel has been in the battles at two gyms, same as staravia.