Freemind has an ebuild but freeplane seems to be its main successor. In this case, the beta version, 1.6.0_26 seems to be highly stable and has a lot of new features so I would not bother with any older versions.
The ebuild should be virtually identical as it is essentially an extraction of an archive, copying the launch program (freeplane.sh) into the path (hopefully with a rename to "freeplane") and creation of a desktop icon.
As far as dependencies and USE flags, the devs at freeplane recommend only using the Sun version of Java (and my experiences have proven them right so far).
*** This bug has been marked as a duplicate of bug 331765 ***
Author: Michael Weber <email@example.com>
Date: Fri Mar 10 12:39:37 2017 +0100
app-misc/freeplane: Initial import (bug 331765, bug 402211).
Package-Manager: Portage-2.3.4, Repoman-2.3.2