  The accuracy of Monte Carlo simulations in reproducing the scientific
performance of space telescopes (e.g. angular resolution) is mandatory for a
correct design of the mission. A brand-new Monte Carlo simulator of the
Astrorivelatore Gamma ad Immagini LEggero (AGILE)/Gamma-Ray Imaging Detector
(GRID) space telescope, AGILESim, is built using the customizable Bologna
Geant4 Multi-Mission Simulator (BoGEMMS) architecture and the latest Geant4
library to reproduce the instrument performance of the AGILE/GRID instrument.
The Monte Carlo simulation output is digitized in the BoGEMMS postprocessing
pipeline, according to the instrument electronic read-out logic, then converted
into the onboard data handling format, and finally analyzed by the standard
mission on-ground reconstruction pipeline, including the Kalman filter, as a
real observation in space. In this paper we focus on the scientific validation
of AGILESim, performed by reproducing (i) the conversion efficiency of the
tracker planes, (ii) the tracker charge readout distribution measured by the
on-ground assembly, integration, and verification activity, and (iii) the
point-spread function of in-flight observations of the Vela pulsar in the 100
MeV - 1 GeV energy range. We measure an in-flight angular resolution (FWHM) for
Vela-like point sources of $2.0^{+0.2}_{-0.3}$ and $0.8^{+0.1}_{-0.1}$ degrees
in the 100 - 300 and 300 - 1000 MeV energy bands, respectively. The successful
cross-comparison of the simulation results with the AGILE on-ground and
in-space performance validates the BoGEMMS framework for its application to
future gamma-ray trackers (e.g. e-ASTROGAM and AMEGO).
