  We present photometric, spectroscopic and polarimetric observations of the
intermediate-luminosity Type IIP supernova (SN) 2021gmj from 1 to 386 days
after the explosion. The peak absolute V-band magnitude of SN 2021gmj is -15.5
mag, which is fainter than that of normal Type IIP SNe. The spectral evolution
of SN 2021gmj resembles that of other sub-luminous supernovae: the optical
spectra show narrow P-Cygni profiles, indicating a low expansion velocity. We
estimate the progenitor mass to be about 12 Msun from the nebular spectrum and
the 56Ni mass to be about 0.02 Msun from the bolometric light curve. We also
derive the explosion energy to be about 3 x 10^{50} erg by comparing numerical
light curve models with the observed light curves. Polarization in the plateau
phase is not very large, suggesting nearly spherical outer envelope. The early
photometric observations capture the rapid rise of the light curve, which is
likely due to the interaction with a circumstellar material (CSM). The broad
emission feature formed by highly-ionized lines on top of a blue continuum in
the earliest spectrum gives further indication of the CSM at the vicinity of
the progenitor. Our work suggests that a relatively low-mass progenitor of an
intermediate-luminosity Type IIP SN can also experience an enhanced mass loss
just before the explosion, as suggested for normal Type IIP SNe.
