In this paper, the (3+1)-dimensional defocusing Gardner-KP equation is investigated again with the help of Lie symmetry method since the results are either incorrect, or incomplete, or misleading in the literature. For the Lie algebra of infinitesimal symmetries spanned by eight vector fields, the one-dimensional optimal system of subalgebra is established by leveraging the fundamental invariants and general adjoint transformation representation. Meanwhile, by virtue of some infinitesimal generators in the optimal system, several symmetry reductions and exact solutions are presented.