$ gemmi reindex -h
Usage:
  gemmi reindex [options] INPUT_MTZ OUTPUT_MTZ
Options:
  -h, --help     Print usage and exit.
  -V, --version  Print version and exit.
  -v, --verbose  Verbose output.
  --hkl=OP       Reindexing transform as triplet (e.g. k,h,-l).
  --no-history   Do not add 'Reindexed with...' line to mtz HISTORY

Input file can be gzipped.