In truenas go into 'system > shell'
Run this
midclt call app.gpu_choices | jq
Take note of the output - you need the pci_slot value, this should look something like 0000:01:00.0 and you need the uuid value which should look something like GPU-ab0fdc2d-e1f7-d2fc-54e0-5b34ff44c3d9. You can copy from shell by highlighting the text and using CTRL + Insert
Then run the following replacing PCI_SLOT with your pci_slot value and GRU_UUID with your uuid value
sudo midclt call -job app.update plex '{"values": {"resources": {"gpus": {"use_all_gpus": false, "nvidia_gpu_selection": {"PCI_SLOT": {"use_gpu": true, "uuid": "GPU_UUID"}}}}}}'