Refer to... http://en.wikipedia.org/wiki/G-force
g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9.80665 m/s², which is approximately equal to the acceleration due to gravity on the Earth's surface.
The symbol g is properly written in lowercase and italic, to distinguish it from the symbol G, the gravitational constant, which is always written in uppercase and italic; and from g, the abbreviation for gram, which is not italicized.
This conventional value was established by the 3rd CGPM (1901, CR 70). The total acceleration is found by vector addition of the opposite of the actual acceleration (in the sense of rate of change of velocity) and a vector of 1 g downward for the ordinary gravity (or in space, the gravity there). For example, being accelerated upward with an acceleration of 1 g doubles the experienced gravity. Conversely, weightlessness means an acceleration of 1 g downward in an inertial reference frame.
The value of g defined above is an arbitrary midrange value on the Earth, approximately equal to the sea level acceleration of free fall at a geodetic latitude of about 45.5°; it is larger in magnitude than the average sea level acceleration on Earth, which is about 9.797 645 m/s². The standard acceleration of free fall is properly written as gn (sometimes g0) to distinguish it from the local value of g that varies with position.
The units of acceleration due to gravity, meters per second squared, are interchangeable with newtons per kilogram. The quantity, 9.806 65, stays the same. These alternate units may be more helpful when considering problems involving pressure due to gravity, or weight.