People interested to boot stage3 minimal bleeding edge gentoo from SD can try VIM2 image Gentoo_8gb_20171130.img.xz
Kernel is vanilla 4.14.2 so: no USB support. HDMI and serial console work.
This is work in progress and it contains some ebuild hacks.
One can ssh to the box u:root p:1234 or u:user p:1
As usual, with gentoo, one is supposed to tune and build own device.
I can assist but this is by no means official and rather playground.
https://forums.gentoo.org/index.php is a good start
Using different dtb from boot/dtb this image may boot on s905(x) as well.
No support for accelerated graphics.
For example: I also built a desktop where one can choose MATE or enlightenment or LxQt GUI.
Kernel is default khadas 4.9.40 for that build.
I can upload desktop image too but it is big and it is even more WIP (e.g. LxQt currently crashes).
Thanks exclusively to @balbes for patient help on various forums and 1:1.