Where are they
The tarballs can be found in the normal place.
This is meant to be just what you need to boot, the disk won't expand itself, it won't even get networking info or set any passwords for you (no default password).
This tarball is suposed to be the base you generate more complex images from, it is what is going to be used by Openstack's diskimage-builder.
The primary things it does is get you a kernel, bootloader and sshd.
This was primarilly targeted for use with openstack but it should work with amazon as well, both use cloud-init.
Network interfaces are expected to use dhcp, a couple of other useful things are installed as well, syslog, logrotate, etc.
By default cloud-init will take data (keys mainly) and set them up for the 'gentoo' user.
I'll be posting about the work being done to take these stages and build bootable images. At the momebt I do have images available here.