In this work, the quasi-3D hyperbolic shear deformation theory (quasi-3D HSDT) is utilized to examine the dynamics of thick rectangular plates reinforced with rectangular nanofillers known as graphene nanoplatelets (GNPs). Agglomeration of the GNPs is incorporated and the mechanical characteristics like shear, elastic, and bulk moduli, Poisson's ratio, and density are analysed according to the mixture along with the Eshelby-Mori-Tanaka approach. Hamilton's principle is hired to derive the solving equations, the Navier approach is hired to present an analytical solution in the spatial domain, and the Newmark method is hired to provide an approximate solution in the time domain. The relevance of the dynamic response and the natural frequencies of the plate on several parameters are explored such as dispersion pattern and the GNPs percentage and agglomeration parameters. It is discovered that for a specific GNPs percentage, growth in the amount of agglomerated GNPs leads to lower natural frequencies and higher dynamic deflection. Meanwhile, for a specific mass fraction of the agglomerated GNPs, growth in the volume of clusters brings about higher natural frequencies and lower dynamic deflection.