GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. AVIF gives the smallest files of any widely supported image format. GIF is at home in short looping clips and reactions; AVIF is the usual choice for the smallest images for websites.
The defaults suit most files. Under Options you can set the quality, aim for a target file size and resize the result.
If the GIF is animated, only its first frame is converted and the result is a still picture. The AVIF encoder is a WebAssembly module that downloads the first time you convert to AVIF and is cached afterwards.