The 'Scale' filter that is currently available does seem to use the GPU, but it's shuffling data back and forth to the CPU, very inefficient. Running 1 dual-output render in VoPro 0.7 is currently slower than running 2 simultaneous renders with Voukoder 13.
'ZScale' is even worse, as it doesn't seem to use the GPU at all.
On the other hand, the speed advantage of 'scale_cuda' should be substantial when it becomes available.
For reference, here are some FFmpeg command lines I've used for scale_cuda with AVI/h.264/h.265 file sources:
Properly fed, all 3 variants will max out the GPU encoder with minimal CPU usage.
An even more powerful feature of FFmpeg is the ability to use multiple graphics cards. Hopefully both GPU filtering (e.g., scale_cuda) and the ability to assign a job to a particular GPU will be added in the future.