"After the 1972 Munich Massacre, GSG 9 was formed to combat the growing threat of terrorism. Due to a problematic military history, GSG 9 grew from the German Federal Border Guard Service/Police. GSG 9 specializes in mitigating crisis situations, such as hostage-taking, kidnapping, terrorism, and extortion." — Siege In-game description
The Grenzschutzgruppe 9 der Bundespolizei (Border Protection Group 9 of the Federal Police), commonly abbreviated GSG 9, is a German counter-terrorism and special operations unit. It was created as a result of the Munich Massacre where Germany's law enforcement found itself severely compromised due to their failure to rescue the hostages and stop the terrorists as well as the country's historic relationship to Jews and Israel.
The patch used in-game to represent the GSG 9 is the patch of the Bundespolizei, which the GSG 9 is a part of.
Design-wise, all GSG 9 Operators feature personnel wearing combat gear over civilian clothing, contrary to the full battledress that GSG 9 personnel universally wear in real life.
The design may have been influenced by the dressing choices that some SEK personnel use in real life, i.e. wearing combat gear over civilian clothing, which is done to conserve time when reacting against an emergency.