yes, it's based on Gentoo
we use profiles & overlays
since Gentoo/portage only provides a package manager (emerge), not overall board tooling, we provide higher level commands like build-packages & build-image to initialize a cross-compiler environment for a board, build all the packages for it, and generate bootable disk images.
yes, we write ebuilds all the time, and send changes to Gentoo. you can look at Gentoo's main overlay tree and see plenty of @
chromium.org or @
google.com authors in it.
-mike