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Gamergate (GamerGate) began in August 2014.
Observers have generally described Gamergate as part of a long-running culture war against efforts to diversify the traditionally male video gaming community, particularly targeting outspoken women.
Harassment and threats have spread to many other venues, many of them outside those in which media fans generally participate.
The Title "Gamergate"
The original title for Gamergate was the IRC channel name "quinnspiracy." Near the end of August, Adam Baldwin (the actor who portrayed Jayne Cobb in Firefly) used the Twitter hashtag "Gamergate," and that title took off.   
Baldwin's August 2014 description of Gamergate was that of a backlash against political correctness, and that it started a discussion "about culture, about ethics, and about freedom." 
The "ethics" referred to a preconceived notion that only white males are truly devoted to games and can write knowledgeably about them, so anyone who does not fit that profile but is interested in writing about gaming must pass an "ethics" screening. Women games journalists were assumed not to have earned proper credentials through years of gaming and technical know-how but were given the job in exchange for sexual favors or were someone's wife or girlfriend. Non-whites were assumed to have benefited from affirmative action.
A DescriptionFrom Wikipedia:
The campaign of harassment was coordinated in IRC channels and online forums such as Reddit, 4chan, and 8chan by an anonymous and amorphous group that ultimately came to be represented by the Twitter hashtag #Gamergate. The harassment included doxing [to "drop docs" on an anonymous user, revealing her identity, phone number and address], threats of rape, and death threats as well as being related to a mass shooting threat in protest of an invited speech featuring Sarkeesian. Gamergate has been described as a manifestation of a culture war over gaming culture diversification, artistic recognition, social criticism of video games, and the gamer social identity. Some of the people using the Gamergate hashtag allege collusion among feminists, progressives, journalists and social critics, which they believe is the cause of increasing social criticism in video game reviews. Some hashtag users have said the goal for their actions is to improve the ethical standards of video game journalism. These concerns have been widely dismissed by commentators as trivial, based on conspiracy theories, unfounded in fact, or unrelated to actual issues of ethics. Users of the hashtag launched email campaigns targeting firms advertising in publications of which they disapproved, asking them to withdraw their advertisements. 
For a much longer, detailed description of Gamergate, see: Gamergate controversy - Wikipedia, the free encyclopedia, Archived version for a very thorough history and description.
Some DiscussionOne opinion:
What is Gamergate? As far as I see things, Gamergate is a hashtag on Twitter that originated in a harassment campaign against prominent industry members that was co-opted by people who are upset about videogame journalism ethics. It is now a confusing mess of people using the legitimacy of the hashtag to further an agenda of harassment, a lot of unsubstantiated conspiracy theories and a well-intended group of people trying to raise concerns about journalistic standards. While I think continuously evaluating the way industry members interact is important, using something that started as and continues to act as a harassment campaign purely as signal booster only serves to weaken a message that would be far better and more effectively made without attaching the negative connotations of the hashtag to it. 
Gamergate Transformative Fanworks and Activities
There is evidence that transformative works were created by people on both sides of Gamergate. Many vids were created trying to explain the events that had lead to Gamergate becoming an international news story, with commentators from different sides of the argument often having differing views. One thinly veiled RPF fic, depicting the rape of one of the main characters in the Gamergate debacle, was for a short time available for sale on Amazon. As the narrative around Gamergate became a wider discussion of sexism in the gaming industry, some female creators were derided as SJWs. Poet Elisa Chavez was criticized for her tone of voice, her technical poetry skills and her knowledge of gaming when her poem went viral. 
- Gamergate controversy entry at Wikipedia.
- Foz Meadows, Rape Culture in Gaming. June 11, 2012. Defines rape culture and discusses the Anita Sarkeesian scandal.
- Elisa Meléndez, What it's like for a girl gamer: harassment in the gaming world. In Slate, 2012-08-13.
- Amy O'Leary, Sexual harassment in online gaming stirs anger. In the New York Times, 2012-08-02.
- Tasneem Raja, Why It Sucks to Be a Woman in the Video Game Industry. Mother Jones, Nov. 27, 2012. Thousands of women working in the video game industry are coming forward with stories of vicious sexism they've faced on the job.
- Aja Romano, How female gamers and comic fans fight real-life sexism online. Daily Dot, July 28, 2012. Discusses the Readercon scandal and quotes from high-profile female fans, with many links.
- Anita Sarkeesian's Feminist Frequency Blog. Ms. Sarkeesian is working on the "Tropes vs. Women in Video Games" project (first episode here -- don't watch the videos on the sidebar that aren't by FeministFrequency... the others are all by people making fun of her and trolling her.). When she put up a page asking for contributions to fund the project, she asked for $6000 and received over $60,000. She also received (and continues to receive) numerous death threats, rape threats, and her Wikipedia page was repeatedly vandalized with racial slurs and images of graphic pornography... all before she even started the project. In August 2014, Ms. Sarkeesian notified police and moved out of her home after a series of harassing twitters indicated that the sender knew her address.  She was featured in the January 15, 2015 edition of ABC's Nightline.
- As an example of the way women are treated in the gaming industry itself, the DOS attack on gaming platform company SendGrid after developer Adria Richards called a couple of co-workers out on Twitter for some blue remarks. Richards was subsequently fired. See also feminist blogger Amanda Blum's take in How We All Lost.
- About Feminism (published in May 2014) is an open letter detailing routine bullying and harassment toward women in the tech industry in general.
- "Women Are Being Driven Offline": Feminist Anita Sarkeesian Terrorized for Critique of Video Games. Democracy Now!, October 20, 2014. Discusses misogyny in gaming and the origins of Gamergate in the Zoe Quinn "scandal". WebCite
- Colin Campbell, Gaming’s toxic men, explained. Polygon, July 25, 2018. Not just more reportage on what is happening, but interviews with "writers and academics who have studied and published useful work on the problem of misogyny and racism in gaming and in popular entertainment. Most have experienced harassment and abuse from toxic gamers."
- Gaming's #MeToo Moment and the Tyranny of Male Fragility by Laurie Penny, wired.com on Sep 6th, 2019
- The sexist crusade to destroy game developer Zoe Quinn by Aja Romano, Archived version at The Daily Dot.
- Gaming’s summer of rage - The Boston Globe, Archived version
- GamerGate: Part I: Sex, Lies, and Gender Games - Reason.com, Archived version
- Happy Anniversary #GamerGate, Love Adam Baldwin, Archived version
- Talia Levin, "The Fetid, Right-Wing Origins of 'Learn to Code'. The New Republic, February 1, 2019.
- Who has the right to call themselves a "gamer", and the definition of who a gamer is, is not a matter of dedication to playing or designing video games; it has a distinct cultural image and identity. Many devoted video game enthusiasts refuse to be associated as "gamers", cf. Dennis Scimeca's Why I Can't Call Myself a Gamer Anymore, Salon, January 1, 2014: and Simon Parkin's If You Love Games, You Should Refuse To Be Called a Gamer, New Statesman Dec. 9, 2013.
- Rami Ismail (ramiismail.com), Archived version, 2014
- A new low: One of Zoe Quinns harassers was selling rape fanfic about her on Amazon, 19 December 2014, accessed 17 September 2018.
- TL;DR - Elisa Chavez Doesn't Understand #Gamergate
- 'The word "troll" feels too childish. This is abuse'. Interview with Sarkeesian in The Guardian, 2015-08-29.
- Juju Chang & Katie Yu, ABC News, When Jumping Into Gamergate Turns Into Fearing for Your Life.