You can download it from here (CustomerSource or PartnerSource login required).
You can read about the new features coming in my Dynamics GP 2018 RTM Feature of the Day series of posts.
The Support team will be running a New Feature series of posts starting on 5th December. These blogs will give a comprehensive overview of the features along with excellent content, documentation, and videos, as they have done in the past.
I'll be going hands on with the new version in a series of posts, probably starting as soon as the download is finished; the posts will start with the installation of the new version, before moving onto covering the new functionality.