Journaling is a fairly new feature of FreeBSD, and as such, it is not very well documented yet. You may however find the following additional references useful:
A new section on journaling is now part of the FreeBSD Handbook.
This post
in freebsd-current by gjournal(8)'s developer, Paweł Jakub Dawidek <pjd@FreeBSD.org>.
This post
in freebsd-questions by Ivan Voras <ivoras@FreeBSD.org>.
The manual pages of gjournal(8) and geom(8).
All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.