Abstract: The Gas-cooled Fast Reactor (GFR), a fourth-generation nuclear system, faces multi-physics coupling challenges due to its high-enrichment core and gas-cooling features. This study develops an RMC-ANSYS-coupled methodology for pin-type GFRs, focusing on a 2 MW modular core. The workflow integrates Monte Carlo-based neutronics (RMC) and thermal-mechanical analysis (ANSYS), enhanced by dynamic geometry mapping to address gas coolant convection and non-uniform expansion. Key geometric deformations, including fuel rod ellipticity and grid eccentricity, were simplified with relaxation-optimized convergence. Results show convergence within two iterations (keff σ=0.00016), revealing core deformation feedback (-623 pcm) as the dominant reactivity mechanism (25× Doppler effect). Deformation-induced fuel displacement toward radial reflectors elevated local power/temperature, validating methodology conservatism. Sensitivity analysis of a 7-assembly "Soccer Reactor" demonstrates anisotropic behavior: 10% radial grid plate expansion reduced keff by 7, 662 pcm (sensitivity -0.73), while axial fuel expansion caused 3, 564 pcm loss (-0.34). Reflectors showed 3× higher axial than radial sensitivity. Grid plate deformation emerged as the critical reactivity driver, establishing foundational insights for deformation compensation and Gen IV GFR safety design. This approach advances traditional density-equivalence methods through explicit geometric distortion modeling.