DISCLAIMER: Can't contribute to wiki + i'm not dealing with grknight again -> filed bug willing to test configuration. https://wiki.gentoo.org/wiki/AMDGPU#Kernel is missing firmware for AMDGPU on AMD PITCAIRN GPUs (https://i.imgur.com/xjvcZ1U.png). `radeon/{amdgpu_{ce,mc,me,pfp,rlc,smc,k_smc},TAHITI_{uvd,vce}}.bin` is not sufficient on AMD7870.
If you want to edit the wiki, but can't it does not help to paste it to the bug tracker instead. I do not see what we can do for you in the bug tracker and close the ticket.