Calibrating tea dosage
O-Cha.com says that a good ratio is 1g of tea leaves to 30g (i.e. 30mL) of water. 

Also keep in mind that, since Gyokuro is such a delicate tea, you want to brew with water that's between 120º and 140º F (50º to 60º C). This is much, much lower than regular black or green teas.