Of course “GB” downloaded will increase .
As because download is a term that u can relate to (transfer of data from one device to another) commonly with the “means of internet” here.
And as you are calling data from firebase realtime database so some internet are required for the transfer of data from the database to the user or (if user writes something then some internet is used for writing into the database and storing them). and hence GB downloaded will increase each time ur user read from or write in some data to the database
(if user write some data in database it will increase both “GB stored” and “GB downloaded”)
DOWNLOAD :- copy (data) from one computer system to another, typically over the Internet.[From google]