Free fall and forced gravity drainage are established production mechanisms, which contribute to significant oil production in naturally fractured reservoirs, commonly 30-60% original oil in place. Accurate and proper implementation of gravity drainage performance, as an exchange term, in dual porosity simulators is a critical issue. A numerical model of gas-oil gravity drainage was developed and solved analytically for some simple cases and also numerically for more complicated forms of capillary pressure and relative permeabilities. In this article, three famous transfer functions in the literature in a comparative study were checked by a developed numerical model. The result revealed that transfer functions studied here predict less accurate results in comparison with the numerical model and using constant matching parameters cannot resolve this issue because problems arise from simple treatment of time dependent parameters, such as capillary pressure and relative permeability.