I know I've seen zodiacal light, especially at my old club's best dark-site. However, I'm pretty sure I've yet to see the gegenschein. How good does the seeing have to be to be able to see that?
On the Bortle Dark-Sky Scale, it takes the very darkest skies - class 1. These will be very, very dark.
The "seeing" doesn't have to be especially good, of course, since the gegenschein is an extended feature and is essentially unaffected by atmospheric refraction. Poor seeing degrades the resolution of observations, but it has no effect on the brightness.