This work presents the sensitivity assessment of gallium nitride (GaN) material-based silicon-on-insulator fin field effect transistor by dielectric modulation in the nanocavity gap for label-free biosensing applications. The significant deflection is observed on the electrical characteristics such as drain current, transconductance, surface potential, energy band profile, electric field, sub-threshold slope, and threshold voltage in the presence of biomolecules owing to GaN material. Further, the device sensitivity is evaluated to identify the effectiveness of the proposed biosensor and its capability to detect the biomolecules with high precision or accuracy. The higher sensitivity is observed for Gelatin (k = 12) in terms of on-current, threshold voltage, and switching ratio by 104.88%, 82.12%, and 119.73%, respectively. This work is performed using a powerful tool, three-dimensional (3D) Sentaurus Technology computer-aided design using a well-calibrated structure. The results pave the way for GaN-SOI-FinFET to be a viable candidate for label-free dielectric modulated biosensor applications.