  We report on NuSTAR observations of the Intermediate Polar GK Persei which
also behaves as a Dwarf Nova. It exhibited a Dwarf Nova outburst in 2015
March-April. The object was observed in 3-79 keV X-rays with NuSTAR, once at
the outburst peak, and again in 2015 September during quiescence. The 5-50 keV
flux during the outburst was 26 times higher than that during the quiescence.
With a multi-temperature emission model and a reflection model, we derived the
post-shock temperature as 19.2 +/- 0.7 keV in the outburst, and 38.5 +4.1/-3.6
keV in the quiescence. This temperature difference is considered to reflect
changes in the radius at which the accreting matter, forming an accretion disk,
is captured by the magnetosphere of the white dwarf (WD). Assuming that this
radius scales as the power of -2/7 of the mass accretion rate, and utilizing
the two temperature measurements, as well as the standard mass-radius relation
of WDs, we determined the WD mass in GK Persei as 0.90 +/- 0.06 solar masses.
The magnetic field is estimated as 4*10^5 G.
