  Gamma-ray astrophysics in the MeV band is an exciting field in astronomy due
to its potential for multi-messenger astrophysics. It has, however, remained
under-explored when compared to other wavelengths. One reason for this
observational gap is the difficulties with measuring these high-energy photons
and the requirement of large amounts of detection material. In this work, we
investigate the usage of large-volume GAGG scintillators for use as a
calorimeter in future MeV telescopes. We developed a $5\times5$ array
calorimeter utilizing $1\times1\times6 \ \mathrm{cm}^3$ GAGG crystals with
onsemi C-series SiPM readout. We tested the calorimeter at the High Intensity
Gamma-ray Facility (HIGS) with monoenergetic beams ranging from $2-25 \
\mathrm{MeV}$. Finally, we also investigate larger $1\times1\times8 \
\mathrm{cm}^3$ crystals and characterize their response across their depth when
their surface treatment is either polished or frosted.
