Composite structures encounter different types of imperfections and defects during the installation, working environment, and fabrication process. In the present research, a 3D-poroflexibility theory has been presented to study the bending responses of the functionally graded graphene nanoplatelets reinforced composite (FG-GPLRC) axisymmetric circular/annular sector plates on the elastic substrate. For modeling elastic substrate, horizontal friction force and the elastic foundation with five elastic parameters have been formulated. For obtaining the exact displacement and stress/strain responses of the current structure, a discrete singular convolution integration method (DSC-IM) has been utilized. Also, the outputs of the current research have been verified with the results of COMSOL Multi-physics software. Consequently, the outcomes demonstrate that GPL’s weight fraction, in-plane boundary condition, radial and circumferential initially stresses, and Biot's coefficient have a remarkable impact on the bending responses of the circular/annular sector plates. Also, when the normal compressive stress increases, the negative effect from normal tensile stress on the bending characteristics of the structures is more remarkable than the positive effect from normal compressive stress. Also, the effect of GPL’s weight fraction on the stress and displacement fields is more considerable at the higher value of the opening angle of the current composite structure.