No putting your cars transmission in neutral will not save any gas while going down hill.
All you need to do when going down hill is to release your foot from the gas pedal and your transmission should downshift on it's own.
If you try to put your transmission in neutral while the car is moving, assuming it is an automatic then you could damage your transmission.
Now if it's a manual then all you would have to do is push in the clutch and then put it in neutral.
But even then all you need to do is downshift to a lower gear when descending down a hill.