@romin The thing is, although GIF is technically a lossless format, in practice it's unable to store enough colours in one image to be useful, so GIF encoding is more often than not lossy.

Most GIF encoders dither the crap out of input images to simulate more colours per image than GIF is able to handle, with the result being a crap quality image, missing colours and a bad looking dither.

PNG and APNG replace the GIF format entirely and don't have bizarre colour limitations.

Due to the difficulty of storing noise losslessy, converting GIF's to APNG's will sometimes result in a larger APNG, but I'm sure you can afford the extra storage space.