Introduced in Ghostbusters II, it is explained in the script that the Giga Meter is a gauge designed to measure psychomagnatheric energy in GeV's (Giga electron volts). The prefix Giga, correctly pronounced 'gi-gah' not 'guy-gah' measures 1 billion and electron volts (eV) are a unit of kinetic energy. It is used several times primarily by Egon to examine the site where Dana Barrett's baby carriage stopped, the Manhattan Museum of Art, the abandoned subway tunnel, and lastly the positive energy rallied by New Yokers during the Lady Liberty walk (shot cut very short).
The Giga Meter is built out of a Power Scrubber 'N' Buffer with assorted electronics and spinning lights inside a plastic dome. The moving ears on top of the Giga Meters are made out of Realistic Stereo Microphones.
The Giga Meter was a rental prop built and owned by Modern Props. As such, it has shown up in several other films and television programs, some of which include:
Teenage Mutant Ninja Turtles: Operation Blueline (1990)
The meter is used as a "Gloomsday Device" to get rid of all signs promoting the new rail system in California.
Star Trek VI: The Undiscovered Country (1991)
The meter is briefly in a montage where Starfleet officers are scanning every uniform on the ship for traces of Klingon blood. In this scene, the pivoting microphone "ears" of the meter are missing, but the digital display on the top was still visible as well as the "scanning orb" on the bottom of the device.