The MBA's at Kodak's marketing department probably figured that G sounded nice. Doesn't need to mean anything.
Or, they had 15 focus groups with professional photographers to figure out which letter would make them more likely to buy the films. The photographers in turn told them to stuff it, that they should just put the money in film R&D. (I have a professional photographer friend who was focus-grouped to determine what box design would make him more likely to buy Portra films.)
E100S was replaced by E100G, E100SW was replaced by E100GX.