$ gemmi merge -h
Usage:
  gemmi merge [options] INPUT_FILE OUTPUT_FILE
  gemmi merge --compare [options] UNMERGED_FILE MERGED_FILE
Options:
  -h, --help             Print usage and exit.
  -V, --version          Print version and exit.
  -v, --verbose          Verbose output.
  --write-anom           output I(+) and I(-) instead of IMEAN.
  -b NAME, --block=NAME  output mmCIF block name: data_NAME (default: merged).
  --compare              compare unmerged and merged data (no output file).
  --print-all            print all compared reflections.

The input file can be either SF-mmCIF with _diffrn_refln or MTZ.
The output file can be either SF-mmCIF or MTZ.