I'm not exactly sure where Gentoo sits with Raspberry Pi compatibility, but best of luck!
If you take a look at ArchLinux and, more specifically Debian's documentation on using aarch64/amd64, you'll find some hints.
Michael Stapelberg has some notes, and there is the Debian Raspberry Pi 3 Wiki that helps explain some of the new architecture, including how to get WiFi working. The current image comes with WiFi, so you'll have to look at instructions for images prior to the 2018 one.
I know that you're on Gentoo, but I'm curious if you could follow a similar set of instructions, which is to pull compile this module, and provide a missing .txt file (/lib/firmware/brcm/brcmfmac43430-sdio.txt).
The steps are documented here but consist of:
- Compiling your kernel with the Broadcom drivers
- Providing the missing
.txtfile:/lib/firmware/brcm/brcmfmac43430-sdio.txt- This paste is the current version as of June, 2018.
- This is sourced from Raspbian
firmware-brcm80211*.deb, so grabbing it from here is a 'better' way of getting the latest version.