"Gun Shot Residue". The particles left over on ones firing arm and clothing after shooting a firearm. GSR can also stand for Galvanic Skin Resistance, Galvanic Skin Response, General Service Representative, General Service Routine/Request, General Staff Requirement, General Support-Reinforcement, Generic Space Robot, Geo-Spatial Referencing (ViaVis Mobile Solutions, Inc.), Gigabit Switch Router (Cisco), Give a Shoot Ratio (polite form), Gland Steam Regulator, Global Search and Replace, Global Shared Resources, Global Signature Register, Government Service Representative (Sprint), Government Social Research (UK), Government Spares Release, Grand Sport Racing, Great Southern Railway, Ground Surveillance Radar, Group Service Representative, Group Strategic Review, and as mentioned by me, Gun Shot Residue (forensics). There are also several other enteries in Urban Dictionary for GSR most of which referr to it at a "Grissom Sara Romance" from the T.V. show C.S.I. Although you will only hear GSR reffered to as Gun Shot Residue and never as a Grissom Sara Romance on the show, as slang GSR can stand for both.
From one forensic to another: "There's GSR all over the deceased's shirt."