  In the framework of detector development, Monte Carlo simulations play a key
role in the evaluation of the expected performance and the full understanding
of the behavior in beam conditions. In particular, a software which simulates
the response of the detector to the particle passage is mandatory to test
different setups and solutions, such as geometries, fields, voltages etc. and
to understand the test beam data. For gas trackers, existing softwares, such as
GARFIELD, perform a very detailed simulation of the physical processes but are
also CPU time consuming. For the new cylindrical GEM tracker of BESIII, a
faster code which models the results obtained from GARFIELD and adapts them to
the experimental data, collected in several test beams, has been written. It
reproduces the behavior of a planar triple-GEM under different working
conditions and, when completed, it will be inserted in the official code of
BESIII. A description of the procedure, based on different components
(ionization, diffusion and magnetic field, avalanche multiplication, signal
induction and readout) will be given and its results will be compared to the
GARFIELD simulations and to the experimental data.
