Harmony JDK can be installed using Gentoo native package manager, portage.

At this moment, binary ebuild for the M3 stable snapshot is available:

Instruction to try out

$ echo 'PORTDIR_OVERLAY="/usr/local/portage"' >> /etc/make.conf

$ mkdir -p /usr/local/portage/dev-java/harmony-jdk-bin/files
$ mv harmony-jdk-bin-0.02.551077.ebuild /usr/local/portage/dev-java/harmony-jdk-bin
$ mv harmony-jdk-bin.env /usr/local/portage/dev-java/harmony-jdk-bin/files

$ ebuild /usr/local/portage/dev-java/harmony-jdk-bin/harmony-jdk-bin-0.02.551077.ebuild digest

$ echo 'dev-java/harmony-jdk-bin ~x86' >> /etc/portage/package.keywords

$ emerge harmony-jdk-bin

$ java-config -L
$ java-config --set-user-vm=harmony-jdk-bin
$ java -version