The subject of "what is RM's media gallery" and "where is RM's media gallery" has been pretty well beaten to death, but let me try to clarify further anyway.
RM's media gallery is not a file on your hard disk. It's not even a folder or a sub-folder tree on your hard disk. Rather, it's a table in your RM database. Your RM database is an SQLite database with many, many tables. There is a table called the MediaTable in your RM database that could be said to be your Media Gallery.
You can view RM's Media Table directly via programming with SQLite. But for most users, the best way to see RM's Media Gallery is via Lists->Media gallery. Using the Lists->Media gallery facility provides a slightly indirect view of your media gallery, but using Lists->Media gallery does provide a pretty good view of your media gallery and it does not require any SQLite programming on your part. Indeed, RM users do not have to do any SQLite programming at all.
RM's MediaTable does not include any media files. Rather, it includes links to all your media files and your media files are stored on your hard disk as standard Windows files such as JPG, PNG, PDF, etc. RM includes a tool to fix broken media links under Lists->Media gallery.
In addition to its links to media files, RM also includes links between the media files and other items in RM such as people, families, facts, sources, citations, and places. These other links are called tags. When you highlight a file under Lists->Media gallery, in addition to showing you the location of the file on the hard disk, RM also shows you all the tags for the file - for example that the file has been tagged to the birth fact for John Doe.
A file which has been linked into RM's media gallery does not have to be tagged to any items in RM. For example, could link an image of a birth certificate to RM's media gallery without tagging the image to anyone's birth fact at the same time. You can include tags at the time you link files into RM's media gallery or you can go back and add the tags later.
Media tags are not stored in RM's MediaTable. Rather, they are stored in a different RM table called the MediaLinkTable. RM joins the two tables together and if you are so inclined you can view the two tables directly with SQLite. But you don't need to use SQLite because Lists->Media gallery displays information for you from both tables in a single integrated screen.