I found it somewhat unintuitive and unpublished, so I thought I’d post my findings here.
Nuke’s brand new GPU support is only currently set up to run GPU operations on the viewport. It will not return them to the next node in the chain.
This is accross the board in all 3 hidden GPU nodes that come with Nuke 5.2 and appears to be built into the IOp calling mechaism (the closed source part of Nuke). So for all those with visions of GPU accelerated compositing… not yet.
I cannot really intuit the design intentions to the level that I can say if this will change in future versions. The one really odd part is the hidden node: BlockGPU. It makes no sense to have a node that blocks the usage of the GPU engine in a tree, if the only way to use the GPU engine is to view a node that uses the GPU engine directly. And secondarily… it doesn’t block the node if you view it directly.
Anyhow, to me it looks like they got just enough code in there to accelerate their new GPU LUT functionality, and left it at that. That’s probably why the GPU nodes are hidden rather than exposed. Maybe in Nuke6.0?