The amount of gas you need to drive 300 miles will depend on your miles per gallon that your vehicle gets.
For example if your car or truck gets 15 miles per gallon it would take you 20 gallons of gas to drive 300 miles.
If your gas mileage was 28 miles per gallon then it would take you around 11 gallons of gas to drive 300 miles.
If your car gets 38 miles per gallon then you would need around 8 gallons of gasoline.
You take the miles per gallon and multiply that by the amount of gallons of fuel that your gas tank holds or the amount of gasoline you think you'll need.
Then you can find out the amount of gasoline it will take to drive the distance you want to drive.