reader2maildir downloads fresh items from your Google Reader account, and places them into a local Maildir. Seen marks are propagated both ways, so if you read an article locally, it will be marked as read remotely, and vice versa.
So by using reader2maildir, you can read your Google Reader items in Gnus, and the seen marks will be synchronized.
Put the .rb's in a directory somewhere.
Create a file named ~/.reader2maildir in your home directory, which
is a YAML configuration file.
username: your username@gmail.com
password: your password
maildir: full path to the maildir you want to use
Each time you run reader2maildir.rb, fresh items will be downloaded
from Google Reader and placed in subdirectories under maildir
corresponding to the feed. Articles which are locally marked as read
will be marked as read remotely, and vice versa.