Usually in elevators, high rise buildings, hotels etc G is Ground Floor or sometimes it might be a star. I have also seen S for Street level or L for Lobby. B for Basement. P for parking. If multiple parking levels P1, P2, P3 etc going down.

Probably basement.

Basement Level

Remember that the USA labels the floors of a building differently from the rest of the world. The pattern P3 P2 P1 G 2 3 is common with subterranean parking starting at the lowest level first and the second floor above ground or lobby level. Another pattern is SB B L M 2 3 ... 12 14 15 with a sub-basement, basement, lobby, mezzanine located below the upper floors and a missing thirteenth floor. In the rest of the world, the pattern G 1 2 3 is more common with the first floor above the ground/lobby level.

yeah, basement :)