If images in your gallery are blurry, there are some variants for solving this.
If it’s Fixed Columns Gallery Type, this type of gallery adapts to screen resolution. That’s why if you have bigger resolution than you set up in the Gallery Settings, thumbnails will stretch for larger screens. In your case, perhaps, you set small value of thumbnails. To improve its quality , go to the Gallery Settings, then increase values of settings “Image width” and “Image height”