Open an .ase or .aseprite file in your browser and export the animation as a GIF. No account, no upload to a server - the file is decoded and re-encoded on your own machine.
Open BitRasterFree, runs in the browser, no account needed.
Frame durations come from the Aseprite file, so the GIF plays at the speed you authored. GIF has one-bit transparency, so a pixel is either fully opaque or fully clear - the export lets you set where that threshold falls.