Density functional theory (DFT) has been widely employed to study the gas adsorption properties of surface based or nanoscale structures. However, recent indications raise questions about the trustworthiness of some literature values, especially in terms of the DFT exchange-correlation (XC) functional. Using hydrogen adsorption on metalloporphyrin-incorporated graphenes (MPIGs) as an example, we diagnosed the trustworthiness of DFT results, meaning the range of expected variations in the DFT prediction of experimentally measurable quantities, in characterizing the gas adsorption/desorption thermodynamics. DFT results were compared in terms of XC functionals and vibrational effects that have been overlooked in the community. We decomposed free energy associated with gas adsorption into constituting components (binding energy, zero-point energy, and vibrational free energy) to systematically analyze the origin of deviations associated with the most commonly adopted DFT functionals in the field. We then quantify the deviations in the measurable quantities, such as operating temperature or pressure for hydrogen adsorption/desorption depending on the level of approximations. Using chemical potential change associated with gas adsorption as a descriptor, we identify the required calculational accuracy of DFT to predict the room-temperature hydrogen storage material.