Any auto parts store has spark plug ramp gauges. It is a progressive ramp that runs from .020" to i think .100". You slide that in between the insulator and the electrode on your spark plug to determine the space in between, the spark gap. Spark plugs that are bought from the auto parts store for your car are pre gapped to the spec.
yes, different spark plugs have different heat ranges lengths diameters and all kinds of other fun stuff. Gap is determined by combustion chamber design, the type of coil and how much power the engine makes.