  The currently developing space-based gamma-ray telescope GAMMA-400 will
measure the gamma-ray and electron + positron fluxes using the main top-down
aperture in the energy range from ~20 MeV to several TeV in a highly elliptic
orbit (without shading the telescope by the Earth and outside the radiation
belts) continuously for a long time. The instrument will provide fundamentally
new data on discrete gamma-ray sources, gamma-ray bursts (GRBs), sources and
propagation of Galactic cosmic rays and signatures of dark matter due to its
unique angular and energy resolutions in the wide energy range. The gamma-ray
telescope consists of the anticoincidence system (AC), the converter-tracker
(C), the time-of-flight system (S1 and S2), the position-sensitive and
electromagnetic calorimeters (CC1 and CC2), scintillation detectors (S3 and S4)
located above and behind the CC2 calorimeter and lateral detectors (LD) located
around the CC2 calorimeter. In this paper, the capabilities of the GAMMA-400
gamma-ray telescope to measure fluxes of GRBs from lateral directions of CC2
are analyzed using Monte-Carlo simulations. The analysis is based on off-line
second-level trigger construction using signals from S3, CC2, S4 and LD
detectors. For checking the numerical algorithm the data from space-based GBM
and LAT instruments of the Fermi experiment are used, namely, three long
bursts: GRB 080916C, GRB 090902B, GRB 090926A and one short burst GRB 090510A.
The obtained results allow us to conclude that from lateral directions the
GAMMA-400 space-based gamma-ray telescope will reliably measure the spectra of
bright GRBs in the energy range from ~10 to ~100 MeV with the on-axis effective
area of about 0.13 m2 for each of the four sides of CC2 and total field of view
of about 6 sr.
