  We construct Holstein-Primakoff Hamiltonian for magnons in arbitrary slowly
varying spin background, for a microscopic spin Hamiltonian consisting of
ferromagnetic spin exchange,Dzyaloshinskii-Moriya exchange, and the Zeeman
term. The Gross-Pitaevskii-type equation for magnon dynamics contains several
background gauge fields pertaining to local spin chirality, inhomogeneous
potential, and anomalous scattering that violates the boson number
conservation. Non-trivial corrections to previous formulas derived in the
literature are given. Subsequent mapping to hydrodynamic fields yields the
continuity equation and the Euler equation of the magnon fluid dynamics. Magnon
wave scattering off a localized Skyrmion is examined numerically based on our
Gross-Pitaevskii formulation. Dependence of the effective flux experienced by
the impinging magnon on the Skyrmion radius is pointed out, and compared with
analysis of the same problem using the Landau-Lifshitz-Gilbert equation.
