Dataflow and PowerBI Gateway - Sourcing data dynamically

If you're trying to connect to a single source that can have many databases (such as SQL Server), then you can use the SQL Server connector without defining a database to connect to and then define your own M query logic as to what table you'd like to query from which specific database inside of the server.

Dataflows is flexible enough in this matter, but it also relies on your own requirements and the logic that you wish to apply.

The SQL Server connector without defining the database will use the Sql.Databases function. If you prefer to write your own M code and more of a code-first approach like in a notebook you can also take that route with Dataflows. 

Hope this helps.