Not good. Gasoline is extremely volatile, has less energy than diesel, has no lubricity, cetane is the fuel rating for compression ignition and adding gasoline may negatively impact burn characteristics and temperatures in the cylinders. If you want "thinning" or concerned about cloud point at extremely low temperatures, use #1 fuel oil or kerosene. Kero has less energy (but much more than gasoline) and no lubricity, but it is recommended in older GM diesels up to 25% for extreme cold weather operation.