That would not completely de-gas it as the one way valve has spring/ air tension to hold it closed. The primary force holding the valve closed is the air pressure in the resevior side (example 2000 psi). You must over come that pressure (at least 2001 psi) to open the valve and then begin filling. When you stop pumping the pressure is equal and the valve is closed. When you pump, there is a little bit more pressure on the hose side, and it goes in the resevoir, then the valve closes again, pressure equalling out again. That equal pressure is what helps keeps your fill plug from popping out under pressure. When you open the bleed valve, you lower the pressure on the hose side, by increasing the volume (as the system is no longer closed). Once the pressure is lower on the hose side, the valve will not open and de-gas. That takes the pressure off the fill plug and now it can be removed quite easily. Otherwise your gun would empty everytime you tried to fill it, regardless of the method (hand pump or tank) you used. Degassing tools overcome this by mechanically holding open the valve and allowing all the air/CO2 to escape.
Nathan