You have to use -d to decompress:

Your command would be:

gunzip -d db-20180518060048.sql.gz

You have to use -d to decompress:

Your command would be:

gunzip -d db-20180518060048.sql.gz

Edit due to comment:

To test validity of your archive you can run

gunzip -t db-20180518060048.sql.gz

-t --test test compressed file integrity

The correct is only no output. Any other means that your archive is damaged.