There is no letter for Gar
There is no rune poem for Gar. It is a Northumbrian Rune.
Gar is the final rune of the Anglo-Saxon/Northumbrian Futhorc. As it falls out of the neat division of rows of eight letters each, it stands outside of the usual ordering. The name is usually translated as spear.
Gar is, as always, the cosmic blank slate. All things and No things expressed as n/one