  A systematic method is developed to study classical motion of a mass point in
gravitational gauge field. First, the formulation of gauge theory of gravity in
arbitrary curvilinear coordinates is given. Then in spherical coordinates
system, a spherical symmetric solution of the field equation of gravitational
gauge field is obtained, which is just the Schwarzschild solution. In gauge
theory of gravity, the equation of motion of a classical mass point in
gravitational gauge field is given by Newton's second law of motion. A
relativistic form of the gravitational force on a mass point is deduced in this
paper. Based on the spherical symmetric solution of the field equation and
Newton's second law of motion, we can discuss classical tests of gauge theory
of gravity, including the deflection of light by the sun, the precession of the
perihelia of the orbits of the inner planets and the time delay of radar echoes
passing the sun. It is found that the theoretical predictions of these
classical tests given by gauge theory of gravity are completely the same as
those given by general relativity. From the study in this paper, an important
qualitative conclusion on the nature of gravity is that gravity can be treated
as a kind of physical interactions in flat Minkowski space-time, and the
equation of motion of mass point in gravitational field can be given by
Newton's second law of motion.
