Type Social Media
Tumblr is a microblogging platform and social networking website that was founded by David Karp and Marco Arment. Now Tumblr is currently owned by Yahoo! Inc. The service allows users to post multimedia, gifs, and other content to a short-form blog were their pages can be view on their dashboard, they can "reblog" or "heart" other user's posts. 
In 2007, Tumblr was founded originally by Karp's own personal earnings as a software consultant for it's parent site, UrbanBaby. It got its' name "Tumblr," which as inspired by the word "tumblelog," a term David used for micro-blogging when he was 19.
On May 16, 2013, Yahoo! may be seeking to acquire Tumblr for as high as $1 billion, according the the tech news site AllThingsD reported and cited multiple anonymous sources which are close to the situation. Later, on May 19, multiple tech news blogs, including AllThingsD, reported that both companies approved Yahoo's $1.1 billion acquisition of Tumblr. This was confirmed as official on May 20.
Social Justice Warriors
The presence of social justice blogging on Tumblr is extensive, spanning over a wide range of issues and topics, ranging from religion, racism, and gender equality to ablism, homosexuality, and classism among many others. Tumblr has a stereotype of being a social justice warriors haven, one blogger from Tumblr, name Communismkills, has experience harassment and doxxing from legions of social justice warriors. 
Role in Gamergate
Fake Tweet #1: https://archive.is/oF1PP]
The first one, Twitter messages only allows a grand total of 147 characters and only allows messages as long as 140 characters.
Fake Tweet #2: https://archive.is/uVEjf]
Second Tweet, the hate message which mentions GG was posted nine months before the screenshot. Said screenshot was posted on Tumblr on May 14, 2015. Secondly, there are 270 days before May 14 was August 17, 2014. So the message was posted approximately August 17 (Adam Baldwin coined the term "Gamergate" on August 27. This post was made before Gamergate existed yet mentions Gamergate by name. - So it is fake. Lastly, the post also uses the term "TOP WHORE 2015" but the message was posted in 2014.
This is an on-going project to get pro-Gamergate information into Tumblr and to expand it's bases outside of Twitter, 8chan, and other established GamerGate hubs to reach people who would not otherwise hear their side.
This operation came out to recruit more people to the Gamergate cause, the brief objectives is to swell Twitter and Reddits' GG members and to recruit newcomers.