Class G airspace is the only form of uncontrolled airspace in the National Airspace System. In Class G, air traffic control does not have authority to provide air traffic separation services. Unless otherwise specified, Class G airspace begins at the surface and extends upwards to 14,500 feet MSL or until it contacts controlled airspace - usually Class E airspace. However, it is worth noting that nearly the entire United States is designated so that Class E airspace begins at either 700 or 1,200 feet MSL.
In the image below, the "fuzzy" side of the blue border indicates an area where class E airspace begins at 1,200 feet AGL. Therefore, inside of this area class G ends at 1,199 feet AGL. Outside of this area, class E begins at 14,500 feet MSL - meaning that class G extends to 14,499 feet MSL.