Running gfix without a command (or an invalid command) results in the following screen of helpful information being displayed:

invalid switch --help
please retry, specifying an option
plausible options are:
  -activate      activate shadow file for database usage
  -attach        shutdown new database attachments
  -buffers       set page buffers <n>
  -commit        commit transaction <tr / all>
  -cache         shutdown cache manager
  -full          validate record fragments (-v)
  -force         force database shutdown
  -housekeeping  set sweep interval <n>
  -ignore        ignore checksum errors
  -kill          kill all unavailable shadow files
  -list          show limbo transactions
  -mend          prepare corrupt database for backup
  -mode          read_only or read_write
  -no_update     read-only validation (-v)
  -online        database online <single / multi / normal>
  -prompt        prompt for commit/rollback (-l)
  -password      default password
  -rollback      rollback transaction <tr / all>
  -sql_dialect   set database dialect n
  -sweep         force garbage collection
  -shut          shutdown <full / single / multi>
  -two_phase     perform automated two-phase recovery
  -tran          shutdown transaction startup
  -use           use full or reserve space for versions
  -user          default user name
  -validate      validate database structure
  -write         write synchronously or asynchronously
  -z             print software version number

 qualifiers show the major option in parenthesis