GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. QOI is lossless and extremely quick to read and write. GIF is at home in short looping clips and reactions; QOI is the usual choice for game textures and pipelines that need very fast encoding.
QOI holds a single still image, so an animated GIF does not stay animated. The defaults suit most files. Under Options you can resize the result.
If the GIF is animated, only its first frame is converted and the result is a still picture. QOI stores every pixel and the alpha channel exactly; the encoder is a small WebAssembly module that downloads the first time and is cached.