I’m happy to have discovered an official arm port in progress. There is an effort already underway to port Gentoo to the ARM architecture. That means there will be external resources I can learn from along the way. Translation: I don’t have to make all the mistakes first – someone else might have already done so.

More good news is that there is an arm use-flag. In brief that means that you can instruct Gentoo to build the system for the ARM processor.

Additionally some of the resources which might come in handy here would be:
How to install Gentoo without a livecd.
Installing Gentoo from an existing Linux distribution