Miles per gallon gasoline equivalent refers to the fuel efficiency of electric vehicles that compares how far a car can go with how much energy is in a gallon of gasoline. This measurement is misleading as efficiency for gasoline engines is radically different from electric motors. The measurement is an attempt to compare apples to oranges, as the challenges of fuel efficiency for a heat engine-powered car are completely different from the challenges of energy efficiency for an electric electric vehicle.
This result is misleading because kWh implies end use energy. Technically, gasoline is part of end use energy (it's a secondary fuel), but gasoline's energy is measured as heat not work. This gives electric vehicles an artificially good efficiency (usually by about a factor of 3). This idea is explained in more detail on the page energy loss.
To get an actual number for our MPGe, someone measures how many kWh the electric vehicle uses to travel 100 miles. This is then converted to the final MPGe measurement. For example, if an electric vehicle uses 29 kWh to travel 100 miles:
Note that this is roughly 3x a normal fuel efficiency for a car (38 mpg). That's because an internal combustion engine has waste heat that accounts for 2/3 of the energy in the car!