Zeolitic imidazolate framework-8 (ZIF-8) has a "gate-opening" framework with narrow pore apertures that swing open by reorientation of 2-methylimidazolate (MelM) linkers enforced by guest adsorption. The present study aimed to employ free energy analysis to provide insight into the mechanism of the adsorption-induced structural transition that results from the reorientation of the MelM linkers. We combined experimental Ar adsorption at cryogenic temperatures with grand canonical Monte Carlo simulations to determine the free energy profiles as functions of the rotational angle of the MelM linker (theta(IM)) and bulk gas pressure. We also estimated the energy fluctuation of the system, which is crucial to discussing the structural transition from a metastable state. The results from the free energy analysis, for example, at 91 K, suggest the following conclusions: A gradual reorientation of the MelM linkers up to theta(IM) = 10.5 degrees occurs with increasing gas pressure that is followed by a spontaneous structural transition to theta(IM) = 25.5 degrees during the adsorption process (gate opening), and then, during the desorption process, an equilibrium structural transition occurs with the opposite reorientation of the MeIM linkers from theta(IM) = 25.5 degrees to theta(IM) = 10.5 degrees (gate closing).