6. why was it necessary to empty the gasoliner
necessary to empty the gasoline from the tanks after auguste piccard's dive?

Need to empty the gasoliner from the tanks after Auguste Piccard's dive:

To explore the ocean, he used the vessel which required tanks with gasoline to sink to great depths. An object placed in water experiences an uplift force called a buoyant force which may hinder sinking if the object’s force is less.  

Since the vessel on its own was lighter than the seawater, the tanks with gasoline increase the force of the vessels so that it can sink to facilitate exploration. Emptying the gasoline was necessary to allow the vessel to come to the surface so that the men can exceed the vessel back to land.