As far as I know, g forces are an equivalent measurement of pressure. IE, 1 G is equivalent to pressure exerted from just existing on earth within its gravity field.
In piloting, the problem in question is the centripetal force generated by turning at extraordinarily high speeds, which in turn can treat you to equivalent amount of forces equal to many times the force of earth's gravity, and thus may be measured in G's; but it's not actually caused by gravity in and of itself.