GridFS is a specification within MongoDB that allows for storing and retrieving files that exceed the BSON-document size limit of 16 MB. DreamFactory's GridFS service uses a MongoDB connection to create a Remote File Service, so you can interact with as you would with any other remote file service, such as AWS S3 or Azure Blob Storage.
Even though it’s a file storage service, you'll notice that the service config tab looks very similar to a MongoDB service. Here you will need to enter in any MongoDB specific connection parameters.
The database you select in the config will be the database where the two GridFS collections are located. These will be by default, fs.files and fs.chunks. Leaving the Bucket field empty will result in the default collections being created, however, you can choose another name for the collection to replace the "fs" portion of the collection(s) if you wish.