This "sport" is very popular on the Texas coast. Never have done it myself, but have seen it on a few videos. The guys on the videos have a big flatbottom boat with a generator and lights mounted on the front of the boat. They cruise around in the shallows looking for them under their lights. Looks like fun and a good way to get a mess of flounder for the freezer.
Get on the internet, click on video, and type in "flounder gigging" and hit search. That's how I found them.