I have been suffering exactly from the same problem on my iPhone since iOS7, I think. Now I am on iOS9, but the problem was still there.
After going through several forums, I found out a good workaroud was to create 2 entries for the same Gmail account:
1. an entry to sync my emails choosing "Other" instead of "Google" and then configuring the account by hand (this would solve the grayed out problem)
2. an entry to sync contacts and calendars choosing "Google"
Now I tried tigran-a suggestion, after creating a single "Google" account for emails, contacts and calendars and then changing the language to English through my Gmail account web interface (General Settings): this solution would also work!
However, I would have liked to find a final solution to the problem, instead of a workaroud!
At last, today I have been able to solve this long-lasting mystery, al least in my case.
In my inbox I had several folders to organize my emails, say folder1, folder2, folder3, etc. Such folders appeared with their regular name both in my iOS mail client and in Thunderbird (the email client I'm using on my PC); however, in the Gmail web interface, the name of one of those folders was prefixed by INBOX/ for some strange reason unknown to me (say INBOX/folder3). After removing the INBOX/ prefix from the name, the iOS problem was fixed!
Hope this solution can apply to someone else.