Open an animated GIF in your browser and export its frames as a packed spritesheet, with padding and extrusion. Nothing is uploaded to a server.
Open BitRasterFree, runs in the browser, no account needed.
A GIF's palette and its one-bit transparency come across as they are. If the source was resized somewhere along the way its pixels may no longer sit on a whole grid - the import screen's pixel-fixing pass measures that and rebuilds at native resolution.