GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. JPEG XL gives excellent quality for its size. GIF is at home in short looping clips and reactions; JXL is the usual choice for high-quality photo archives.
The defaults suit most files. Under Options you can set the quality and resize the result.
If the GIF is animated, only its first frame is converted and the result is a still picture. The JXL encoder is a WebAssembly module that downloads the first time you convert to JXL and is cached afterwards.