The expression is "elephant in the room". The gorilla bit comes from some recent insurance commercial.
"Elephant in the room" is an English metaphorical idiom for an obvious truth that is being ignored or goes unaddressed. The idiomatic expression also applies to an obvious problem or risk no one wants to discuss.[1]
http://en.wikipedia.org/wiki/Elephant_in_the_roomIt is based on the idea that an elephant in a room would be impossible to overlook; thus, people in the room who pretend the elephant is not there have chosen to avoid dealing with the looming big issue.
The "see the elephant" meant to see the terrors of the unknown. There were mammoth bones found in the Western Plains back in the 1830's, but no one ever saw any elephants. People who crossed the plains said they were off to "see the elephant". Elephants were sen as large and scary, the way we now see T Rex's.
The Republican Party was referred to as the elephant as a symbol scare the slaveholders and their allies in the Eastern business elite. The elephant destroyed the Whig Party coalition of Capitalists, militarists and slaveholders.
The gorilla has a different origin in a different expression:
"800 pound gorilla" is an American English expression for a person or organization so powerful that it can act without regard to the rights of others or the law.[1] The phrase is rooted in a riddle:
"Where does an 800 lb. gorilla sleep?"
The answer:
"Anywhere it wants to."
This highlights the disparity of power between the "800 lb. gorilla" and everything else.
The term can describe a powerful geopolitical and military force, or, in business, a powerful corporate entity that has such a large majority percentage of whatever market they compete within that they can use that strength to crush would-be competitors. (The metaphor includes an inherent bit of hyperbole; the highest weight yet recorded for an actual gorilla is 600 lb (270 kg).)
The metaphor has been mixed, on occasion, with the metaphor of the elephant in the room, as in TV advertisements by the financial firm AXA Equitable broadcast in 2010.[2] In 2011 former Arkansas governor Mike Huckabee referred to the healthcare plan instituted by former Massachusetts governor Mitt Romney as "the 800-pound elephant in the room".[3]