  We study from first principles the magnetic, electronic, orbital and
structural properties of the LaMnO3 doped with gallium replacing the Mn-site.
The gallium doping reduces the Jahn-Teller effect, and consequently the
bandgap. Surprisingly, the system does not go towards a metallic phase because
of the Mn-bandwidth reduction. The Ga-doping tends to reduce the orbital order
typical of bulk antiferromagnetic LaMnO3 and consequently weakens the
antiferromagnetic phase. The Ga-doping favors the G-type orbital order and
layered-ordered ferromagnetic perovskite at x=0.50, both effects contribute to
the formation of the insulating ferromagnetic phase in LaMn1-xGaxO3.
