Chose Polygoon Tool> Before drawing Polygon Tool Set Your Isolation in the top toolbar option as 40mills or whatever your requirement is then draw the Polygon after it turned to dotter chose name toolbar and click on the polygon and set name to GND then click the ratsnet button and see the magic! you can also change the isolation setting after its been pour by using info tool and click on the side of polygon tool!
Isolation is the setting you want to give to tell that how much distance will be between your ground pour and routing layers!
Regards,
Punit