A coolant temperature gauge is a means to measuring the core temperature of the engine.
More on Coolant Temperature Gauges - Diesel Trucks & More
In most applications, it is sufficient to use a temperature gun in order to create a basic idea of the comparison, but you really need a coolant temperature gauge for a perfect picture of what's going on. A temperature gun has limitations: it can only read the surface temperature of an engine, and therefore is insufficient when trying to truly ascertain the temperature of the engine. These guns are unable to measure the coolant's temperature through the engine's aluminum or cast iron component, therefore the surface temp will always read as cooler than the coolant inside the engine, which is also called engine "core temp". A typical temperature differential of between 5 and 25 degrees exists between the gauge and the temperature gun.