In this paper, the Glass Fiber Reinforced Polymer (GFRP) composite samples are explored in order to evaluate their feasibility and adaptability for use in future metasurface antenna application. Multi-layer GFRP composite samples are fabricated with a proportionate ratio of resins and fiber using Vacuum Assisted Resin Transfer Molding (VARTM) technique. N type to waveguide (WR-187) adapter specially designed for electrical characterization of these GFRP composite samples is used. Thru-Reflect-Line (TRL) calibration technique is used for the test setup, and scattering parameters of these GFRP samples is measured by using the manufactured adapter along with the sample holder on a two port Vector Network Analyzer (VNA). Relative permittivity and dielectric loss tangent of GFRP composite samples are computed using Nicholson-Ross-Weir (NRW) and New Non-Iterative conversion methods. The comparative analyses of both methods showed a very good agreement between them. The GFRP sample with the lowest relative permittivity is short listed for its possible application in future metasurface antenna.