Gasification with supercritical water is an efficient process that can be used for the valorization of biomass. Lignin is the second most abundant biopolymer in biomass and its conversion is fundamental for future energy and value-added chemicals. In this paper, the supercritical water gasification process of lignin by employing reactive force field molecular dynamics simulations (ReaxFF MD) was investigated. Guaiacyl glycerol-β-guaiacyl ether (GGE) was considered as a lignin model to evaluate the reaction mechanism and identify the components at different temperatures from 1000 K to 5000 K. The obtained results revealed that the reactions and breaking of the lignin model started at 2000 K. At the primary stage of the reaction at 2000 K the β-O-4 bond tends to break into several compounds, forming mainly guaiacol and 1,3-benzodioxole. In particular, 1,3-benzodioxole undergoes dissociation and forms cyclopentene-based ketones. Afterward, dealkylation reaction occurred through hydroxyl radicals of water to form methanol, formaldehyde and methane. Above 2500 K, H2, CO and CO2 are predominantly formed in which water molecules contributed hydrogen and oxygen for their formation. Understanding the detailed reactive mechanism of lignin’s gasification is important for efficient energy conversion of biomass.