Guided-mode resonances (GMRs) in periodically perturbed dielectric slabs enable ultra-narrow spectral features with small footprints. Still, achieving high- Q operation on thin-film lithium niobate (TFLN) without degrading material quality remains challenging. Here, we numerically study a silicon dioxide overlay grating on an unetched LiNbO 3 slab that implements an etchless GMR metasurface compatible with wafer-scale lithium niobate-on-insulator (LNOI) processing. Using a reproducible single-cell finite-element workflow, we map geometry to performance through hierarchical sweeps of period ( P ), overlay thickness ( h g ), and slab thickness ( h wg ). The best-performing device ( P = 820 nm , h g = 80 nm , h wg = 320 nm ) exhibits a near-unity reflectance peak around 1.5 µm with full width at half maximum of ∼ 0.11 nm , corresponding to Q ≳ 1 0 4 . The etchless architecture preserves the LN surface, avoids plasma-induced loss, and supports electro-optic tuning via the Pockels effect. These results provide design rules for narrowband filters and label-free refractometric sensors on TFLN and establish a scalable route to ultra-narrow, voltage-tunable metasurfaces.