GitLab primarily stores configuration data in its (externalize) database. Actual project-data — the stuff moved via git push, git clone and the like — is stored on the GitLab server's local filesystems. In order to foster quicker recoverability in the event of the loss of an EC2 instance, this data is hosted on an EFS share. The make_gitlab_EFS.tmplt.json template creates the requisite persistent file-server service while the EC2-related templates configure GitLab's accessing/use of that file-server.
Currently, the only content stored on the EFS share is the ${GIT_HOME}/git-data directory (typically /var/opt/gitlab/git-data). By default, gitlab stores file uploads (user- and project-avatars, file-attachments, etc.) in the /var/opt/gitlab/gitlab-rails/uploads directory (in versions prior to 9.5, a different path was used). This directory is moved onto the EFS share, as well.
In the case of a rebuild-event, EFS allows the replacement instance to come up with all of its contents in the expected locations.