Class G airspace includes all airspace below 14,500 feet MSL not otherwise classified as controlled. Class G airspace is typically the airspace very near the ground (1,200 feet or less), beneath class E airspace and between class B-D cylinders around towered airstrips. It isn’t charted, and it exists wherever Class A, B, C, D or E doesn’t. airspace in the United States. It is most easily found on a sectional map when a fading, thick blue line appears.
In the real world, radio communication is not required in class G airspace, even for IFR operations. It is the only form of “uncontrolled” airspace in the United States.