In practice, the difference usually comes down more to filter quality than to glass vs gel alone.

A high-quality filter of either type should be optically flat and reasonably uniform, so neither is inherently sharper or more even just because of the material. Good gels can perform very well, and many photographers would see little or no difference in normal-sized images.

Placement can matter. A front-mounted filter mainly risks adding flare or stray light, especially if coatings are poor. A rear gel filter sits in the optical path close to the lens and any lack of flatness or warping can reduce image quality. Because glass is more rigid and easier to coat well, high-quality glass filters often have an advantage in resisting warping and controlling flare.

Any filter also slightly alters the optical path, but in normal use this is rarely noticeable because you compose and focus with the filter in place.

So: neither type is automatically sharper or more uniform, but a well-made glass filter often has a practical edge due to rigidity and coatings. With high-quality filters, the real-world difference is often small.