Gloria is Cannibal

The truth of the reality of Gloria is quite obvious with noting some of the factual evidence from the movie.

Francis is very aware of his surroundings and can adequately connect incongruences in his atmospheres. This is depicted in the interaction with the missing girl and the photos where he clearly notates the connection with the man who comes into the store to develop his photos.

He also does indeed perceive things and senses issues prior to their occurrences. He informed his brother Sal (which he refers to as step mother) of the surrounding issue with Vic being interested in him. The results of that truly prove unfortunate.

When one partakes in Cannibalism such as Gloria, the human tissue consumed has a rather adverse reaction which involves progressive neurodegenerative diseases called the transmissible spongiform encephalopathies (TSEs). In particular this is similar to a disease called Kuru which means (shaking death). This completely accounts for Gloria's unique reactions and uncontrollable shaking due to her unusual eating habits.

Reference link: http://scienceblogs.com/neurophilosophy/2008/08/11/the-shaking-death-cjd-epidemic/

Thank you for your time that is all.
P.S. Francis' hallucinations were merely warnings and signs to be interpreted to uncover the truth. (His personal lifestyle and habits were quite unhealthy in themselves and clearly an onset from his childhood experiences.)