Is a gasoline-diesel fuel a good mixture to get more power from a diesel engine?

not at all. that will cause engine damage. gasoline burns at a much lower temperature than diesel fuel so you will get pre ignition. diesel fuel has more useable power in than gasoline. gasoline is rated in octane and diesel fuel is rated in cetane. with diesel fuel the higher the cetane the faster the fuel will burn for a more complete combustion. so if you want more power from a diesel a place to start is a higher cetane diesel fuel. but things like bigger trubos, vgt turbos, injectors, intercoolers have a much bigger effect on power than fuel