If I am asking a silly question, but what exactly is regulator gain?   It is a term at work I keep hearing but I cant place exactly what its supposed to mean.  Our product is fuel modules in cars that contain a fuel pressure regulator which opens and dumps excess fuel back into the reservoir, so what would the gain constitute? 

If your device functions in the usual "control sense", the Gain is similar to "leverage".  The controller for your regulator device takes the difference between the controller's input signal (say that would be the actual fuel pressure discharged from the regulator) and the controller's set point (which would be the desired regulator discharge pressure).  The controller determines relatively how far away the discharge pressure is from the set pressure and calculates a correcting signal (probably) based on the following formula,

Output signal = Gain * ( E + Td + 1/Ti