The gerrie:setup-database command is responsible to setup the database scheme (tables only) for the gerrie:crawl command. The gerrie:setup-database command won`t create the database it selfs. The database have to already exist.
If the database contains tables the command won`t overwrite something. The command checks if a table with the same name already exists (this is done by SHOW TABLES LIKE ...). If yes, the command does nothing and will execute the same procedure for the next table. If the requested table does not exist it will be created (this is done by CREATE TABLE ...).
What does this mean? This means that if you will upgrade from an old version of Gerrie to a newer one and you know that there were database schema changes that this changes won`t be applied to your already existing scheme. Database scheme changes has to be applied by a different way.
In the normal world you can apply the gerrie:setup-database to every existing database which contains various tables already. All tables which will be created by Gerrie as prefixed by gerrie_. If you do not get any tables which got the prefix gerrie_ you can just apply this command to your database.