Getting a couple simple questions answered by you could lead to an actual answer.
1) Are you referring to the phrase "software encoding only" in the Summary section of the Export dialog?
2) Are you exporting something that actually uses the GPU during export like say Lumetri color correction but your system says the GPU is inactive during export of said clip?
If the first is "yes" ... that Summary section phrase only refers to the presence or lack thereof in your CPU of Intel's new QuickSync hardware embedded in some recent CPUs. Which only comes into play during encoding of H 264 codecs. If your CPU has QuickSync and it's enabled, you get a hardware encoding phrase there while exporting to H 264.
Any effects on the GPU Accelerated Effects List will still be processed within Pr using the GPU as the CPU sends data to be processed.
If 2) is your situation ... first, make sure you have something like Lumetri corrections done to a clip. Maybe add a couple other effects from that GPU Accelerated list for good measure.
Then export while watching the system report on resources used to see what work if any was passed to your GPU.
If you have Mercury Acceleration on CUDA in the Project settings but nothing on the GPU Accelerated list is using GPU, then check manually on the Nvidia site to make sure you have the latest drivers.
And ... remember ... the GPU is *only* used while processing things on the GPU Accelerated list ... while such clips are being processed ... and only works as much as the CPU sends stuff for it to work on.
Neil