  We present optical photometry and spectroscopy of the superluminous SN 2002gh
from maximum light to $+204$ days, obtained as part of the Carnegie Type II
Supernova (CATS) project. SN 2002gh is among the most luminous discovered
supernovae ever, yet it remained unnoticed for nearly two decades. Using Dark
Energy Camera archival images we identify the potential SN host galaxy as a
faint dwarf galaxy, presumably having low metallicity, and in an apparent
merging process with other nearby dwarf galaxies. We show that SN 2002gh is
among the brightest hydrogen-poor SLSNe with $M_{V} = -22.40 \pm 0.02$, with an
estimated peak bolometric luminosity of $2.6 \pm 0.1 \times 10^{44}$ erg
s$^{-1}$. We discount the decay of radioactive nickel as the main SN power
mechanism, and assuming that the SN is powered by the spin down of a magnetar
we obtain two alternative solutions. The first case, is characterized by
significant magnetar power leakage, and $M_{\mathrm{ej}}$ between 0.6 and 3.2
$M_{\odot}$, $P_{\mathrm{spin}} = 3.2$ ms, and $B = 5 \times 10^{13}$ G. The
second case does not require power leakage, resulting in a huge ejecta mass of
about 30 $M_{\odot}$, a fast spin period of $P_{\mathrm{spin}} \sim 1$ ms, and
$B\sim 1.6 \times 10^{14}$ G. We estimate a zero-age main-sequence mass between
14 and 25 $M_{\odot}$ for the first case and of about 135 $M_{\odot}$ for the
second case. The latter case would place the SN progenitor among the most
massive stars observed to explode as a SN.
