Updated 64-bit Gentoo Image for RPi3 Released (now also for RPi3B+ and RPi4B)
if you would like to try running Gentoo in 64-bit mode on your Raspberry Pi 3, you might like to have a look at the updated bootable image I've just released, on GitHub, here.
Update 1 September 2019: a new 1.5.1 release of the image is available (now also supports the RPi4!); please see this post for more details.
You can burn the image (~821MiB compressed) to a microSD card (>=8GB), then boot your RPi3 from it directly (the root partition will be automatically resized to fill the card on first boot). Full instructions for download and use are provided on the project's GitHub page.
The image contains a complete (OpenRC-based) Gentoo system (including a full Portage tree, up-to-date as of 9 July 2017) - so you can run emerge operations immediately - and has a reasonably populated userland (Xfce v4.12, LibreOffice v188.8.131.52, Firefox v53.0.3, Thunderbird v52.2.0, VLC v2.2.6, GIMP v2.9.4 etc.) so that you can get productive without having to compile anything first (unless you want to, of course ^-^). Just download, xzcat to a microSD card, and boot!
WiFi and Bluetooth both work, as does sound (via onboard headphone jack, and over HDMI). VC4 acceleration is supported via the mixed-mode vc4-fkms-v3d device-tree overlay / kernel module / Mesa driver, and performance seems reasonable (glxgears 400-1200fps, real-time video playback). The kernel on the image is 4.10.17-v8 from raspberrypi/linux, in pure bcmrpi3_defconfig form.
As of version 1.1.0 of the image, a weekly-autobuild binhost, custom Gentoo profile, and binary kernel package have been provided, making it relatively painless to keep your system up-to-date. Here's a screenshot:
There's also a (sticky) thread about this image on the Gentoo forums, here.
Have fun ^-^
PS this is an updated version of the image that RoyLongbottom used for his 64-bit benchmarks (see this post). For those with significant amounts of work invested in the 1.0.2 image, manual upgrade instructions are also available, here.