Gallium-based liquid metal is a novel lubricant for high-end equipment due to its low friction coefficient and excellent load-carrying capacity. In this work, the stribeck curve, static friction and a series of fixed-speed friction tests for steel self-mated pairs under gallium lubrication are conducted to reveal the lubrication regimes and transition laws. In the sliding speed ranging from 10−8 to 1.4 m/s and the normal load ranging from 2 to 20 N, gallium can only provide boundary and mixed lubrication. Uniformly distributed or island-like distributed gallium-rich films are formed under boundary or mixed lubrication regimes, respectively. The different formation mechanisms are revealed. The failure of the gallium-rich film is mainly related to the average contact stress of the frictional interfaces.