If it is a GSG-5 SD I hope you have checked with ATI to see if it is part of the BATFE recall.
On January 2010 American Tactical Imports Inc received official notification from the Bureau of Alcohol, Tobacco, Firearms and explosives that the original barrel shroud (aka: fake suppressor) supplied with your GSG 5 SD model must be replaced. It has been determined that this shroud is regulated under the National Firearms Act. American Tactical will provide a replacement shroud at no charge for each GSG 5 SD model sold or currently in inventory.
Consumers in possession of a GSG 5 SD model with the original shroud in place on the firearm are now in violation of the NFA. To avoid continued violation of the NFA, ATI asks that all persons in possession obtain a replacement shroud as soon as possible. We anticipate arrival of the new shrouds to begin by the middle of February 2010.
http://dealer.americantactical.us/gsg-recall