if you want to try running Gentoo Linux in 64-bit mode on your Pi-Top, you might like to have a look at the bootable image I've just released, on GitHub (please search there for "gentoo-on-rpi3-64bit", the forum seems to swallow messages with links in the body text).
You can burn the image (~829MiB compressed) to a microSD card (>=8GB), then boot your Pi-Top 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 (be sure to download the Pi-Top variant, as it contains additional drivers for your Pi-Top's battery, screen brightness etc.).
The image contains a complete (OpenRC-based) Gentoo system, 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. Just download, write to a microSD card, and boot to an Xfce desktop!
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.
Courtesy of Rene Richarz's work, the Pi-Top image also contains:
* a patched xfce4-battery-plugin that queries the RPi3's battery over I2C (and displays a gauge in the panel);
* working backlight brightness (communicates with the Pi-Top's controller hub over SPI) with keyboard shortcuts;
* full poweroff on shutdown (ditto); and
* support for playing HDMI audio via any installed pitopSPEAKER units.
There's a 64-bit version of Gordon Henderson's handy wiringpi library (and gpio utility) included on the image too, as the above drivers need it to operate.
If you get a chance to try it, please let me know how you get on!
Have fun ^-^