At first you need to place Media files data source webpart to the page where you want to use Media Gallery - File List. Then you will just use its ID in the File List webpart.
Now if you want to show only certain folder you may need define Where Condition property of Datasource webpart, for example:
FilePath LIKE N'folder/%' AND FilePath NOT LIKE N'folder/%_/%'
Please note 'folder' is folder which is under current media library. This will show files from 'folder' path and not other else.