This is now depreciated to some extent, I am on kernel 4.14.78-gentoo and it is working well. YOu still need to follow the Gentoo WIKI though.
Having run into major problems with an old Nvidia card, I decided to go for a new Dual Head Radeon card. The problem with the Nvidia card was that the drivers had reached a point of not being compatable with my kernel (4.4.6), so updating @world was becoming a nightmare.
The RX470 is a rather nice bit of kit, but does not use radeon drivers, but the newer amdgpu drivers. These are not easily available, certainly not for Gentoo. The AMD website only provides an Ubuntu package.
You don't get much info from lspci
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Device 67df (rev cf)
Nothing 'Human Readable' as regards the chipset anyway
I followed the Gentoo amdgpu Wiki but discoverd there that my card needed kernel 4.7-rc6 or later. I downloaded kernel 4.7.5 from the kernel.org site.You will need the Polaris10 drivers from the /lib/firmware/amdgpu folder.
Followed all the instructions, built the kernel as usual and I got a Kernel Panic!! on the first boot.
Then I read that all VGA settings in the boot loader (I use Lilo) must be removed. Bingo all was well.
With regard to the Xorg settings I removed my old xorg.conf file which the nvidia needed, added the file /etc/X11/xorg.conf.d/20-amdgpu.conf and I have no problem.
Section "Device" Identifier "Device0" Driver "amdgpu" VendorName "AMD" BoardName "RX 470" EndSection
No errors in the Xorg.0.log at all on starting X.
Now if only I could get the framebuffer working for the nice 'emergence' splash Theme.
I Fixed it!
Installed the 4.7.4-gentoo kernel from the ebuilds, set it up as above, and BINGO!! Splashing Eyecandy!!!
This is probably because the kernel.org kernel 4.7.5 does not have the gentoo patches.