Numerical simulations and experimental studies concerning melt flow and primary as well as secondary gas penetration during the filling and the postfilling stages in gas-assisted injection molding of a thin plate with a semicircular gas channel design were conducted. Distribution of the skin melt thickness along the gas-penetration direction was measured to identify primary and secondary gas penetration. Melt and gas flow within the gas channel of a semicircular cross section is approximated by a model which uses a circular pipe of an equivalent hydraulic diameter superimposed on the thin part. An algorithm based on the control-volume/finite-element method combined with a dual-filling parameter technique suitable for the tracing of two-component flow-front advancements is utilized and numerically implemented to predict both melt-and gas-front advancements during the melt-filling and the gas-assisted filling processes. A flow model of the isotropic melt-shrinkage origin combined with a gapwise layer tracing algorithm was implemented to assist the prediction of secondary gas penetration and melt flow in the post-filling stage. Simulated results on the gas front locations at the end of both primary and secondary penetration phases show reasonably good coincidence with experimental observations.