Really this girlfriend seems annoying.
Rather than try to convince her to like video game music, I think she has much larger problems that she needs to address. Were she in one of my classes, then we could discuss in great length her attitudes. But since I don't know her in person, that isn't going to happen.
So, rather than offering video game soundtracks I offer instead three academic articles she might want to read:
Janet Levy, "Covert and Casual Values in Recent Writings about Music" Journal of Musicology, Vol 1 No 1, Winter 1987
Robert Fink, "Elvis Everywhere: Musicology and Popular Music Studies at the Twilight of the Canon" American Music Vol 16 No 2, Summer 1998
Bethany Bryson, "'Anything but Heavy Metal: Symbolic Exclusion and Musical Dislikes" American Sociological Review, Vol 61 No 5, Oct 1996
All three articles are available through JSTOR if your friend's girlfriend has access to it.
But if she is determined to fixate on the music of German nationalism, there isn't much you can do about it.
Note: I say this as a Professor of Musicology