Gamma spec is shorthand for gamma spectroscopy. Gamma spec is the process and equipment used to discern and identify radionuclides which emit gamma radiation. Here's our friend Tom (the computer scientist who enjoys health physics, and who can blame him?) showing us his system. He didn't do very well explaining radioactive decay, but he excels here:
As you can see from his gear, it would be difficult to transport that stuff around after an unintentional radioactive release. Instead, vehicles are usually used with gross detection instruments (can't discern individual radionuclides, just total radiation).
The guys in the video below constructed a mathematical model to be used with gross detection instruments in place of spectroscopy (it needs more work, but interesting start):
There are handheld gamma specs, but they can't discern very low levels of excess radioactivity: