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 $dot{M}$.
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