Gel formation is described by a nonequilibrium self-assembly (SA) mechanism which considers the presence of precursors. Assuming that nonequilibrium structures appear and are maintained by entropy production, we developed a mesoscopic nonequilibrium thermodynamic model that describes the dynamic assembly of the structures. In the model, the evolution of the structures from the initially inactivated building blocks to the final agglomerates is governed by kinetic equations of the Fokker-Planck type. From these equations, we get the probability densities which enable one to know the measurable quantities such as the concentrations of the different components and the dynamic structure factor obtained in light-scattering experiments. Our results obtained are in very good agreement with the experiments. The model proposed can in general be used to analyze the kinetics of formation of nonequilibrium SA structures usually found in biomedicine and advanced materials.