Old Green Gage is the definitive "gage", and often considered the best flavoured of any plum variety.
Compared to the attractive coloring of most plums, the appearance of Old Green Gage fruit is fairly plain - the plums are quite small and a dull green color, turning slightly yellow when ripe. However this is one that you grow for flavor - for eating fresh it is exceptional.
Old Green Gage really benefits from the microclimate of a sheltered south-facing wall or fence to bring out the best flavours.
If the tree sets a lot of fruit after a good spring, be sure to thin the fruitlets, otherwise the plums will be small and have less flavour.