What do the G, L and E avatar attributes mean?
I discovered the answer to this on the Groove Coaster Facebook page.
When selecting an Avatar, a description of that Avatar is displayed at the bottom of the screen, followed by the letters G, L, and E and some numbers. These letters and the values associated with them signify the special attributes for that Avatar as follows: G (GROOVE): This is the initial amount of GROOVE you begin a stage with. L (LENGTH): This refers to the length of the Groove Gauge. E (EXP): This amount modifies the amount of experience points earned when playing stages.