Gill nets are typically illegal for "normal" fishing use... If it is vacuum packed away in your survival gear, hard to prove that you are actively using it.

However, in a survival situation, everything is valid to catch food. As scafool said, would be happy to have someone come along to fine me if I'm in a survival situation! ;-)

Gill nets can be used both in active and passive mode... The mesh size does determine what it will/won't catch, but it can be used much like a seine net in active mode.