For GPU, F# has prior art in targeting it, a notable one is
https://developer.nvidia.com/blog/jet-gpu-powered-fulfillmen...I believe it is mostly about intrinsic libraries and developing the code to stick to the idioms of using such libraries; in the context of Clef, maybe the author of this infrastructure is aiming at close interaction between the compiler frontend, a standard GPU programming library and the Clef backend, but I can't really comment on this.
What Clef seems to be doing is:
* bringing semantics that are needed for native platform targeting, expressed with idioms familiar to F# developers (through the type system and the lowering during compilation stages, retaining target specific semantics)
* making the broad F# programming idioms applicable to such targets (when applicable obviously)
The website contains a lots of detailed articles about aspects of translating F# constructs to the type of environment this compiler targets, but AFAIR, nothing specific to GPU programming.