This paper focuses on modelling quasioptical elements, such as highly directive Gaussian optic lens antennas (GOLA) and dielectric slabs, using vectorial Gaussian beam expansion and tracking techniques. GOLAs are key components for freespace characterisation benches and are used in THz imaging, sensing and communication applications. The developed modelling tool is employed to model a free-space characterisation bench, which includes a dielectric slab of rexolite positioned between two GOLAs. A Thru-Reflect-Line (TRL) calibration is applied on the simulated datasets to compare the resulting S-parameters with measurements and with an analytical model of a dielectric slab.