Searching transition state corresponding to the diffusion process first, then the barrier is simply the difference between the energy of the transition state and that of the neighbouring minimum structure.
You should specify what exactly energy barrier you're asking about. It doesn't seem to be easy to "calculate" in Gaussian since it's a macroscopic process.
This is a typical energy barrier calculation process. First find the structures of reactants and transition states, and obtain thermodynamic quantities through frequency vibration analysis. For convenience, the keyword of opt freq can be written together, and then the energy difference can be found.