If you have a Chromium OS SDK checkout you can import Gentoo packages and deploy them to a an image built with --no_enable_rootfs_verification, but that is a lot of work as you would have to put your device into dev mode, build your own CrOS image, import, and deploy the Gentoo packages.
Alternatively, you can use crostini to get a linux environment on Chrome OS. There was of building your own container so it is conceivable you could get a gentoo environment to work there, but that would probably be about as much work as building your own CrOS image.