The hydraulic performance of the flexible pavement is defined by the particle size distribution (PSD), thickness, and degree of compaction of the Granular Subbase (GSB) layer. Transportation agencies like the Indian Roads Congress (IRC) and the American Association of State Highway and Transportation Officials (AASHTO) insist that the GSB layer intends to have a minimum in-plane permeability of 300 m/day to attain optimal performance. In addition, the designers should consider the concept of internal stability of granular filters so that a stable gradation is ensured even under the impact of external factors such as seepage and vibrations. In this study, newly proposed GSB gradations were evaluated for their in-plane permeability characteristics and internal stability, and the results were compared with those of the existing GSB gradations. The in-plane permeability characteristics are evaluated experimentally through a large-scale permeameter under different hydraulic gradients. The internal stability of the GSB gradations is evaluated analytically using the internal stability index. The comprehensive test results indicated that the effects of PSD and hydraulic gradients highly influenced the in-plane permeability of the GSB. The open-graded GSB materials are intended to have a higher rate of permeability and a lower internal stability index than the dense-graded GSB materials.