It is based on Gentoo as software can be based on make and gcc. But the user only sees the resulting binary.
AFAIK Portage, Gentoo's package manage, is used only to build the rootfs image. Because Gentoo is really a meta distribution. This distinction is not an empty slogan. You can use Gentoo as a build system to create separate operating system, just like ChromeOS does.
You can use of course use Gentoo directly and prepare tailor built binary packages to distribute them to users. Then it would be a bit more similar to other distributions. But ChromeOS is not an ordinary distribution. It's update model is not really compatible with typical distribution model. It's one of the greatest strengths of the system. You could shoehorn packages meta data, but it would be pointless, because whole image is updated at once.
AFAIK Portage, Gentoo's package manage, is used only to build the rootfs image. Because Gentoo is really a meta distribution. This distinction is not an empty slogan. You can use Gentoo as a build system to create separate operating system, just like ChromeOS does.
You can use of course use Gentoo directly and prepare tailor built binary packages to distribute them to users. Then it would be a bit more similar to other distributions. But ChromeOS is not an ordinary distribution. It's update model is not really compatible with typical distribution model. It's one of the greatest strengths of the system. You could shoehorn packages meta data, but it would be pointless, because whole image is updated at once.