GIF is limited to 256 colours and produces very large files for anything longer than a few seconds. PPM is the simplest image format there is, readable by any programming language. GIF is at home in short looping clips and reactions; PPM is the usual choice for passing images between command-line tools.
PPM has no transparency, so see-through areas of a GIF are filled with a solid colour, white by default; you can change it under Options. PPM 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. PPM stores three bytes per pixel with no compression, so files are large.