You can look at MIAOW which is an open source implementation of the AMD Southern Islands GPU ISA.[0]

The RTL can be found here [1]

The best part is the white paper [2], PDF

I had been looking around for information similar to ECS[3], building a GPU from first principles and MIAOW is one step closer that.

[0] http://miaowgpu.org/index.html

[1] https://github.com/VerticalResearchGroup/miaow

[2] https://raw.githubusercontent.com/wiki/VerticalResearchGroup...

[3] https://www.nand2tetris.org/