  We report on X-ray observations of the Dwarf Nova GK Persei performed by {\it
NuSTAR} in 2015. GK Persei, behaving also as an Intermediate Polar, exhibited a
Dwarf Nova outburst in 2015 March--April. The object was observed with {\sl
NuSTAR} during the outburst state, and again in a quiescent state wherein the
15--50 keV flux was 33 times lower. Using a multi-temperature plasma emission
and reflection model, the highest plasma temperature in the accretion column
was measured as $19.7^{+1.3}_{-1.0}$~keV in outburst and
$36.2^{+3.5}_{-3.2}$~keV in quiescence. The significant change of the maximum
temperature is considered to reflect an accretion-induced decrease of the
inner-disk radius $R_{\rm in}$, where accreting gas is captured by the
magnetosphere. Assuming this radius scales as $R_{\rm in} \propto
\dot{M}^{-2/7}$ where $\dot{M}$ is the mass accretion rate, we obtain $R_{\rm
in} = 1.9 ^{+0.4}_{-0.2}~R_{\rm WD}$ and $R_{\rm in} = 7.4^{+2.1}_{-1.2}~R_{\rm
WD}$ in outburst and quiescence respectively, where $R_{\rm WD}$ is the
white-dwarf radius of this system. Utilising the measured temperatures and
fluxes, as well as the standard mass-radius relation of white dwarfs, we
estimate the white-dwarf mass as $M_{\rm WD} = 0.87~\pm~0.08~M_{\rm \odot}$
including typical systematic uncertainties by 7%. The surface magnetic field is
also measured as $B \sim 5 \times 10^{5}$~G. These results exemplify a new
X-ray method of estimating $M_{\rm WD}$ and $B$ of white dwarfs by using large
changes in $\dot{M}$.
