A class of nonrelativistic magnetosonic soliton, where the charge-separation electric field can be as large as the background magnetic field, is discovered in a strongly magnetized cold plasma, where the electron cyclotron frequency is much larger than the electron plasma frequency. We study how the Mach number of such a soliton is changed by the presence of a gentle background density gradient. An effective Hamiltonian for the soliton trajectory is derived, with which one can show that the soliton Mach number increases as the soliton travels to a background of increasing density, or equivalently, decreasing Alfven speed. The soliton can generally exchange its energy, momentum, and mass with the background plasmas. Our results also show that the soliton may undergo wave breaking at a finite background Alfven speed.