  SPI-ACS/INTEGRAL is one of the most sensitive orbital gamma-ray detectors in
energy range above 80 keV. Since 2002 it registered several thousands of
gamma-ray bursts, including the bursts associated with LIGO-Virgo gravitational
wave events GW 170817 and GW 190425. No dedicated in-flight calibrations were
performed for SPI-ACS/INTEGRAL, complicating estimation of spectral and
energetic characteristics of an event. Using data of GBM/Fermi we perform
cross-calibration of SPI-ACS/INTEGRAL, based on 1032 bright GRBs registered by
both experiments. We find the conversion factor between instrumental counts
from SPI-ACS and energy units from GBM to be dependent on hardness of GRB
spectrum (defined as the characteristic energy value, $E_{p}$) and on location
of a source in spacecraft based coordinate system. We determine the
corresponding analytical model to calculate the conversion factor and estimate
its accuracy empirically. Sensitivity of SPI-ACS/INTEGRAL to detect gamma-ray
transients is also investigated. Using the calibration we re-estimate
energetics of GRB/GW 190425, detected by SPI-ACS/INTEGRAL alone. We constrain
possible range of the characteristic energy $E_{p}$ and isotropic equivalent of
total energy, emitted in gamma-rays $E_{iso}$ for GRB 190425, using the $
E_{p,i} $ -- $ E_{iso} $ (Amati) correlation. The calibration model could be
applied to any transients with energy spectrum, analogous to gamma-ray bursts.
