Facebook removed the ability to 'gate' a Facebook Page based on if a user Likes it. The new policy was announced, and immediately took effect, on August 7th.
For more information, see their
Facebook Graph API v2.1 announcement
post. Specifically:
The liked field will no longer be returned in the page property of the signed_request object for Page Tab apps created from today onwards. For apps created before today, from November 5, 2014 (90 days from today), the liked property will always return true regardless of whether or not the person has liked the page.
I hope that helps answer your question,
Alex