What does a helicopter "gas prod" gauge measure?
The instrument shows the gas generator rpm in the engine. It think you are referring to this scene (screenshot from youtube):

The Bell 412 in the movie uses a pair of Pratt &amp; Whitney Canada PT6T-3BE Twin-Pac turboshafts, which are free turbine engines. The turboshafts used in helicopters consist of,

Compressor

Combustion chamber

turbine
and other components.


The gas producer (from which 'GAS PROD' came) forms part of the turbine. The gas generator forms the first stage that runs the compressor and makes the engine self sustaining. It also runs the oil and fuel pumps. As it is a free turbine, the gas generator stage is not mechanically connected to the next (power turbine) stage. From faasafety.gov:

The turbine section consists of a series of turbine wheels that are used to drive the compressor section and the rotor system. The first stage, which is usually referred to as the gas producer or N1 may consist of one or more turbine wheels. This stage drives the components necessary to complete the turbine cycle making the engine self-sustaining. Common components driven by the N1 stage are the compressor, oil pump, and fuel pump. The second stage, which may also consist of one or more wheels, is dedicated to driving the main rotor system and accessories from the engine gearbox. This is referred to as the power turbine (N2 or Nr).

In some cases, NG is used in place of N1. The gas generator speed is measured in the PT6 engines using a tachometer-generator mounted  on the engine.
From United turbine training Manual:

The tachometer-generator produces an electric current which is used in conjunction with a tachometer to indicate gas generator RPM. The tachometer-generator drive and mount pad are located at the 5 o’clock position on the accessory gearbox and is driven form the internal scavenge pump.