The Gamma Rate Meter (aka: Radiacmeter IM-179 Gamma Rate Meter) was used by Egon during the blasting of Chambermaid's cart (in Sedgewick Hotel) and the battle of Gozer. It is never seen again in any other form of Ghostbusters media. It wasn't even made into a toy like the Ghost Sniffer. It's use in the movie was never noted, but the device is
Real Life Uses
Basically it reads Radiation levels in the area.