Techically, at least in the US, unless you have a license (which tend to start in the 20K range) you can't legally produce a kit.
That being said, most GK manufacturers just fly under the radar. You find some sellers listing kits under generic names to disguise thier origins (like Arrowhead Battleship). Japan I think has the right idea, the Wonderfestival. An event where GK manufacturers can sell thier stuff without fear of reprocussions. And, as has happened, the legit manufacturers get to see whats out there and may buy the GK's masters to mass produce.
The only time Copyright law comes into effect is if you use a copyrighted name. Specific shapes and designs fall under IP laws (Intellectual Property) and are much more confusing. Most IP owners will license out the rights to certain things and let the licensee police things. Every now and again some GK manufacturer will get a C&D (Cease and Desist) and they'll stop producing said kit(s).