As far as I understand, javafx is open-source and supported by OpenJDK. However, Gentoo doesn't package it and if you want to use it, you have to use Oracle's JDK. Please add a javafx useflag which either installs a subpackage or installs it as part of icedtea so users don't have to switch to Oracle's JDK.
*** This bug has been marked as a duplicate of bug 547918 ***