<p>This study investigates the effect of various debonding conditions on the dynamic vibrational behavior of GFRP-reinforced steel structures through numerical simulations alongside experimental tests for a selected group of debonding conditions. Intact and debonded samples were analyzed in a variety of sizes ranging from 5% to 90% of the bonded area, considering four debonding configuration. The first six natural frequencies obtained numerically were used in quantitative statistical analysis and validated against a group of experimental results. The results revealed that natural frequencies constantly declined as the debonding size expands, with the drop becoming significant beyond 25%. Smaller debonding sizes (5% and 10%) exerted a minimal influence on the first two vibration modes (mode 1–2), which makes the early detection difficult using lower order modes. In contrast, higher modes (modes 4–6) exhibited significant reduction in frequency of vibration even for smaller debonding sizes, indicating higher sensitivity to localized stiffness loss. Among the configurations, longitudinal debonding produced the most significant, whereas centrally positioned transverse debonding exhibited a minor effect. The accuracy of the generated statistical models in identifying relevant parameters demonstrated a high coefficient of determination (R²) ranging from 95.7%, to 99.6% for the first six modes. The numerical results were also close to the experimental measurements with ≤ 10% error for the analyzed debonding scenarios. The research advances the assessment of debonding in GFRP-steel structures by quantifying the combined impact of debonding size and configurations on modal response and demonstrating the effectiveness of GFRP in improving the structural performance across vibration modes, in addition to its potential application for corrosion resistance.</p>