if you are running a 'pure' 64-bit Gentoo system on your RPi3 B / B+ (such as my gentoo-on-rpi3-64bit image), you may sometimes find yourself wishing you could just install and run a particular 32-bit package from Raspbian, if it has not yet been added to the Gentoo arm64 tree.
Well, the good news is you can, while still running your 64-bit Gentoo, since mixed-mode userland is permitted on ARMv8! I've just posted a new wiki tutorial, showing how to install a 32-bit Raspbian chroot on your gentoo-on-rpi3-64bit system, then, using this, install 32-bit apps (using apt-get) from Raspbian, and run them alongside 64-bit Gentoo apps on your (64-bit) desktop. The example app I use in the tutorial is Lazarus (but you can obviously install anything you like). Here's a screenshot of 32-bit Raspbian Lazarus running on an 64-bit Gentoo RPi3 B+:
Have fun, sakaki ^-^