Fix the document https://www.firebirdsql.org/pdfmanual/html/gfix-dbverify.html
The option required to fix a corrupted database is the gfix -m[end] command. However, it cannot fix all problems and may result in a loss of data. It all depends on the level of corruption detected. The command is:
gfix -m[end] database_name
Vlad pointed me that there is no "-m" for gfix. The correct should be "-me" (as displayed by Firebird 3's gfix -?).