Hydrogeological and near-surface environmental systems often involve coupled gas, water, heat, and reactive processes in variably saturated heterogeneous porous media, demanding modelling tools that preserve mechanistic process representation. The Geo-Environmental Gas and Contaminant (GEGC) Lab at Carleton University developed GEGCSim, a process-based model designed to support mechanistic understanding of coupled gas transport and reactions in heterogeneous porous media. It is implemented in COMSOL Multiphysics as a coupled setup linking two-phase gas and water flow, multicomponent gas species transport, and heat transfer. Two-phase advection is represented using Darcy-based flow for gas and liquid. Multicomponent transport accounts for advection in each phase, gas-liquid partitioning through dissolution, and diffusion in the gas phase represented using a Stefan-Maxwell formulation. Heat transfer is represented through conduction and convection associated with phase flow. Reactive behaviour is represented through interchangeable source and sink terms, allowing configuration for different reaction networks. Currently, methane oxidation is included as an example, which is represented using Monod-type kinetics with temperature and moisture correction factors. Hydraulic behaviour is described using van Genuchten water retention and relative permeability functions, enabling simulation of capillary barrier effects, moisture redistribution, and dynamic gas-filled porosity. GEGCSim was validated by simulating engineered methane oxidation biosystems (MOBs), which are reactive landfill cover systems used to mitigate fugitive and residual landfill methane emissions, using both laboratory and field-scale studies. The validation reproduced coupled gas distribution, moisture and thermal responses, and methane removal efficiency under different boundary conditions. Beyond methane mitigation, GEGCSim can be configured for other coupled unsaturated zone problems.