NGS Style Manual‎ > ‎- G -‎ > ‎

gram, g

gram is abbreviated g (no period), singular and plural:
            15 g

g is also the abbreviation for for a unit of force, no period:
            g-force
            g meter
            zero g
            1 g; 5.5 g (use g for singular and plural abbreviation of gravity; g's permitted in direct quoted speech)
but     G suit
            G-string

See also METRIC AND INTERNATIONAL MEASUREMENT.