Gallagher teamed up with the former Saatchi & Saatchi copywriter and founder of the new creative agency Rocket Science, Robin Murtouth, and the photographer Simon Nicholls to work on the campaign for free.
It will run for two weeks from 21 November on sites around London. Media planning and buying was donated by Posterscope, and sites by Maiden, bringing the value of the campaign to £250,000.
The posters illustrate the impossibility of distinguishing between real and imitation weapons, and how imitation handguns can be converted into working weapons for as little as £50. One execution reads: "Point blank, even police officers can't spot the imitation."
The GCN chairwoman, Gill Marshall-Andrews, said: "The success of the 1997 ban on handguns is being undermined by a loophole in the law that allows all major gun manufacturers to make exact replicas under licence. The GCN is calling for a ban on the sale, import and manufacture of these imitation weapons."