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.