I'm the first and, until very recently, the only person who has been using Gaea for map making. Jip and others have been using WorldMachine, which is similar. You can see my maps in this thread, all of which have been made using Gaea.
As part of Jip's mapping tournament, I will be doing an introductionary livestream + Q&A on how to use Gaea for mapping. Time and date of this stream will be announced later this month on the forums and in discord. The documentation I've written on my workflow and stuff will be similarly posted around that time.
In the meantime, feel free to send me a message on discord. I don't mind taking an hour or two to talk you through the basics during a videocall. Alternatively, the Gaea quadspinner discord has a lot of information as well.