GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. BMP is read by old Windows programs and simple hardware that understand nothing else. GIF is at home in short looping clips and reactions; BMP is the usual choice for legacy Windows software and embedded devices.
BMP 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.