We present a semiclassical analysis of Dirac electron tunnelling in graphene monolayer with mass gap through a smooth potential barrier in ballistic regime. This 1D scattering problem is formulated in terms of transfer matrix and treated in WKB approximation. For a skew electron incidence this WKB approximation deals with four turning points. Between the first and the second, the third and the fourth turning points two tunnelling domains are observed. Scattering through a smooth barrier in graphene resembles scattering through a double barrier for 1D Schrodinger operator that is 1D Fabry-Perot resonator. The main results of the paper are very simple WKB formulas for the entries of the barrier transfer matrix which explain the mechanism of total transmission through the barrier for some resonance values of energy of a skew incident electron. Moreover, we show an existence of modes localized within the barrier and exponentially decaying away from it and its behaviour depending on mass gap. These are two sets of discrete complex with small imaginary part and real energy eigenlevels determined by Bohr-Sommerfeld quantization condition, above and below the cut-off energy, respectively. It is shown that the total transmission through the barrier takes place when energy of incident electron coincide with a real part of one of complex energy eigen levels. This facts were confirmed by numerical simulations done by the finite element method (COMSOL).