Systems, methods, and devices for determining a neutron-gamma density (NGD) measurement of a subterranean formation that is accurate in both liquid- and gas-filled formations are provided. For example, a downhole tool for obtaining such an NGD measurement may include a neutron generator, a neutron detector, two gamma-ray detectors, and data processing circuitry. The neutron generator may emit neutrons into a formation, causing a fast neutron cloud to form. The neutron detector may detect a count of neutrons representing the extent of the neutron cloud. The gamma-ray detectors may detect counts of inelastic gamma-rays caused by neutrons that inelastically scatter off the formation. Since the extent of the fast neutron cloud may vary depending on whether the formation is liquid- or gas-filled, the data processing circuitry may determine the density of the formation based at least in part on the counts of inelastic gamma-rays normalized to the count of neutrons.