A "metaphor" is a figure of speech in which one thing is compared to another without using any "comparison" words, such as "like". For example, "Her words were a dagger pointed at my heart." Her words were not literally a dagger of course, but her words were like a dagger -- presumably because they hurt me like being stabbed with a dagger.
So yes, "metaphor" refers to things that are said to be similar or alike. This is not an "extended meaning", but the normal meaning of the word. (And to the best of my knowledge, the only meaning.)
"Metaphorical" is just the adjective form of "metaphor", meaning "having to do with a metaphor. Referring to the example above, you might say, "The metaphorical use of the word 'dagger' indicates how much her words hurt."
There is a commonly-used metaphor, "X was the 300-pound gorilla in the room", where "X" is someting that people are trying to ignore or pretend doesn't exist. Like, "The company's profits have been falling for several years. The 300-pound gorilla in the room is the new president, the son of the previous president but obviously incompetent." The idea is that you really have to work hard to ignore a 300-pound gorilla, but everyone does for some reason.
The sentence you quote appears to come from an article in the New York times. http://www.nytimes.com/2016/06/07/science/gorilla-shot-harambe-zoo.html?_r=0 The article is about the gorilla in the Cincinnatti Zoo who was recently shot by zoo officials to prevent him from harming a child who had fallen into his enclosure. The writer says that a subject no one wants to talk about is the ethics of keeping primates in zoos. No one wants to talk about it, i.e. a 300 pound gorilla in the room. The sentence you quote is linking the literal gorilla who was shot to the metaphorical gorilla that people are supposedly trying to ignore.