Annual fuel cost for the gas car is miles divided by miles per gallon, times the gas price. Annual energy cost for the EV is miles times the energy use per mile, times the electricity price. The difference is the yearly fuel gap. Insurance, depreciation, and the purchase price are not in that gap.
12,000 miles at 30 miles per gallon and $3.50 a gallon costs $1,400 a year in gas. The EV cost depends on the kilowatt-hours per mile and the cents per kilowatt-hour you enter. The page subtracts those two yearly bills.
Insurance, maintenance, depreciation, and a home charger are not included. A public fast-charge price is used only if you type that price into the electricity field.