Curtail is a wrapper around tools like pngquant, jpgoptim and oxipng. In lossless mode it optimizes and reorders e.g the compression tables. I know of no tool that does that for video data and I am not even sure that is feasable. Video is not stored as a simple sequence of images.

As a CLI there is gifsicle or imagemagick.

As a gui maybe gifcurry, tupitube or some other 2d animation tool https://en.wikipedia.org/wiki/List_of_2D_animation_software