/usr/bin/gfs2gfs is in gerris 20131206+dfsg-5.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35  | #!/bin/sh
usage()
{
	cat <<EOF
Usage: gfs2gfs [OPTIONS] < OLDFILE > NEWFILE
Converts old Gerris simulation files to the current format.
Options:
        [--help]    display this message and exits
EOF
	exit $1
}
while test $# -gt 0; do
  case "$1" in
  -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;
  *) optarg= ;;
  esac
  case $1 in
    --help)
      usage 0 1>&2
      ;;
    *)
      usage 0 1>&2
      ;;
  esac
  shift
done
sed 's/^ *GtsSurface/GfsSolid {}/g' | \
sed 's/GtsSurfaceFile/GfsSolid/g' | \
sed 's/surface =/solid =/g'
 |