@RJL is the person with the deepest gear knowledge for this type of question. The GPU is not just "an addendum" to the CPU, but a completely different subsystem. And is coded as such, and that differently in every app. Plus H.264 throws another variable in the mix, if that's what you're using for proxies ... though I hope not.
Depending on the particular CPU and GPU, the current versions can have very different usage of the GPU for encoding purposes. And just being an i9 is not necessarily meaning you get more GPU usage, it depends on which i9 CPU you've got.
And what of course is applied for effects ... many things are not actually run through the GPU in Premiere at all. Some things will use the GPU for everything they can, such as Warp and Lumetri, some others on the GPU accelerated effects list.
So ... for say basic encoding of proxies, without any effects, depending on the CPU and what format/codec you're using for the proxies, you may get little to no GPU effort. Or ... more.