The term "Great Debate Community" was first used at least as early as 2014 [though probably before--if anyone has sources for earlier use of this term, or more information on its origins, please cite them] to refer to a loose social network of people on Google+, YouTube, and Google Hangouts centered around discussion and debate of religion (particularly American Christianity) and atheism/ skepticism, and the scientific and philosophical issues related to this controversy. This "community" got its start on YouTube around 2007/ 2008 when a number of now well-known channels (e.g. Thunderf00t, AronRa, potholer54, DonExodus2, cdk007, dprjones, AndromedasWake, along with many others) were launched with the primary goal of debunking Young-Earth Creationist (YEC) arguments, and educating people on the relevant sciences. One could argue that the community of Internet atheists, skeptics, and science promoters which eventually became the GDC goes back even further, as AronRa, Richard Carrier, and a number of others were arguing with Creationists on Usenet and other internet fora during the 1990s, long before YouTube came into existence.
The Great Debate Community took on a more recognizable form in 2013 with the launch of Google Hangouts. Prior to this, direct voice/ video interaction between participants was done mainly through streamed Skype calls or on other platforms like BlogTV. Hangouts provided an easy means of setting up a virtual chat room, sharing the link to bring people, and then streaming the hangout live on the host's YouTube channel with the click of a button. A number of people and groups began running regular live hangouts very soon after the platform launched, including BrianJ1962, Dragnauct, Jade West, Ran Campbell, the Fundamentally Flawed podcast, and the New Covenant Group. The term "Great Debate Community" was also coined around this time, probably by AgentofDoubt. 2014 saw the community expand even further, with many more channels and regular hangout-based shows appearing.
In 2016, Steve McRae created a community page on Google+ called the "Great Debate Community" to serve as a focal point for discussion of issues related to religion and science, which has since grown to over 1,200 members. When the term "Great Debate Community" is used on this wiki, it's most likely referring to this specific group page, rather than the broader community on G+/ YouTube, which can be a bit nebulous and hard to define.