Interband photoexcitation in monolayer graphene can produce a weak gain in the terahertz range by only up to 2.3%, but exciting the surface plasmon polaritons mediates the light-matter interaction, resulting in a giant terahertz gain. Nonlinear carrier relaxation/recombination dynamics and resultant stimulated terahertz (THz) photon emission with excitation of surface plasmon polaritons (SPPs) in photoexcited monolayer graphene has been experimentally studied using optical-pump/THz-probe and optical-probe measurement. We observed the spatial distribution of the THz probe pulse intensities under linear polarization of optical pump and THz probe pulses. It was clearly observed that intense THz probe pulse was detected only at the area where the incoming THz probe pulse takes a TM mode being capable of exciting the SPPs. The observed gain factor is in fair agreement with theoretical calculations.