I guess it can't be done using Feedparser. I needed to do the same thing and I solved it using the imaplib library to change email's flag from "UNSEEN" to "SEEN" after checking the gmail subjects this way:
import imaplib
details = feedparser.parse("https://" + PiEmail + ":" + PiPwd + "@mail.google.com/gmail/feed/atom")
FromEmail = details['items'][i].author_detail.email
Subject = details['items'][i].title.lower()
obj = imaplib.IMAP4_SSL('imap.gmail.com', '993')
obj.login(PiEmail, PiPwd)
obj.select('Inbox')
typ ,data = obj.search(None,'UnSeen')
obj.store(data[0].replace(' ',','),'+FLAGS','\Seen')
I hope it works for you!