Assume diesel and gasser have same torque, compare...

Basic high school physics. Horsepower is work accomplished in a specific amount of time. It does not matter whether the fuel is gasoline, propane, or diesel: The engine that has the higher horsepower will produce more work in a given amount of time than one with lesser horsepower. That is how they assign horsepower numbers. Torque is another matter, and is commonly confused with horsepower.