E would stand for End. On a football field there is a Defensive End that plays on the defensive line, a Tight End that usually plays on the offensive line and is a pass receiver, and a Split End who is a wide receiver.
Until 1965, NCAA mandated it's players to play a one platoon system meaning that one unit played both offense and defense. The E position meant that the player played on the outside or ends of the line. Ends played both DE on defense and TE on offense.
G can stand for goalkeeper in football.
In football terminology, FS means Free Safety, a defensive position.