Slice a spritesheet into frames in your browser and export the animation as a GIF. BitRaster measures the grid for you rather than making you count pixels.
Open BitRasterFree, runs in the browser, no account needed.
Grid detection is a measurement rather than a guess, but a sheet with uneven margins or mixed cell sizes can defeat it - which is why the grid is editable before you commit, and why the preview shows you the slice before anything is created.