The kinetics of diffusion-controlled, stochastically gated biochemical reactions is analyzed within the markovian approximation for stochastic fluctuations of reaction rate. With the use of methods developed in the theory of magnetic field effects on chemical reactions, several general expressions for reaction rate and transient kinetics of geminate and bulk reactions are derived. In particular, it is shown that gating strongly manifests itself not only in steady-state reaction rates but also in the long time tail of kinetics. Specific features of gated reactions in the presence of attractive potential, resulting in the long-lived intermediate State: (cage), are discussed. Two simple markovian models of gating are considered which allow significant simplification of the general expressions obtained. Within these models simple analytical formulas for reaction rate and reaction kinetics are derived and analyzed in detail.