Alright, thanks for the links! I’ll see if I can build ITK with OpenCL and put it in some of our kernels. That should give me an idea if high the efforts are. The OpenCL-related issues you mentioned were mostly solved in CLIJ btw. E.g. we use an image-type-agnostic dialect of OpenCL and have a nicely working memory management.
SYCL sounds very interesting, but I have the feeling it’s not widely adopted by the community yet. I couldn’t find many adoptable code examples…
When it comes to joint efforts for building a GPU-backend that is usable from ITK and ImageJ - count me in! I’d be happy to support these efforts. 
Thanks again for the hints! I’ll keep you posted.
Cheers,
Robert