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We report on X-ray observations of the Dwarf Nova GK Persei performed by {\itNuSTAR} in 2015. GK Persei, behaving also as an Intermediate Polar, exhibited aDwarf Nova outburst in 2015 March--April. The object was observed with {\slNuSTAR} during the outburst state, and again in a quiescent state wherein the15--50 keV flux was 33 times lower. Using a multi-temperature plasma emissionand reflection model, the highest plasma temperature in the accretion columnwas 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 maximumtemperature is considered to reflect an accretion-induced decrease of theinner-disk radius $R_{\rm in}$, where accreting gas is captured by themagnetosphere. Assuming this radius scales as $R_{\rm in} \propto\dot{M}^{-2/7}$ where $\dot{M}$ is the mass accretion rate, we obtain $R_{\rmin} = 1.9 ^{+0.4}_{-0.2}~R_{\rm WD}$ and $R_{\rm in} = 7.4^{+2.1}_{-1.2}~R_{\rmWD}$ in outburst and quiescence respectively, where $R_{\rm WD}$ is thewhite-dwarf radius of this system. Utilising the measured temperatures andfluxes, as well as the standard mass-radius relation of white dwarfs, weestimate 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 isalso measured as $B \sim 5 \times 10^{5}$~G. These results exemplify a newX-ray method of estimating $M_{\rm WD}$ and $B$ of white dwarfs by using largechanges in $\dot{M}$.

An Estimation of the White Dwarf Mass in the Dwarf Nova GK Persei with NuSTAR Observations of Two States