Gill nets used to be a required piece of emergency gear for any planes operating in northern Canada.

Gill nets are meant to hang in open water like a curtain. They have floats on the top edge and weights on the bottom. You can rig them under ice on frozen lakes or hang them from logs.
The mesh is sized so a fish running into it gets stuck just past the gills and can not go ahead or back out of it, the mesh catches behind the gill covers. Gill nets can be very specific about what size of fish you catch, and even what species. Fish which are too small swim through the mesh and fish which are too big don't get far enough into the mesh to get caught.
Gill netting is a passive fish catcher. You set the net and come back later to haul it in. Gill nets kill the fish they catch. It might sound odd but the fish drown in them.

Most of the nets people refer to as gill net are actually seine nets and are meant to strain all the fish out of the water. They are usually much smaller mesh and only the smallest of fish can escape. They can be rigged to trap fish like a pound net, used like a purse net or as a fish trap. You can drag them through the water or set them up as a dip net. Usually you use them actively as a seine. You surround the fish with them and then pull them in like a bag.

Fish nets can be excused as hammocks and as carry bags, but try not to get caught with one rigged with floats weights and lines on it for fishing if you are camping near water.

Now if I was in a survival situation I would be happy to have a game warden appear and tell me I was fishing illegally.

It is odd how the law is different if you are actually in a survival situation.