It seems that a lot of GPU problems revolve around specific versions of drivers. Though NVidia has their own support structure, you can often learn from information reported by others who fold.
15:37:08:******************************* System ********************************
15:37:08: CPU: Intel(R) Xeon(R) W-2135 CPU @ 3.70GHz
15:37:08: CPU ID: GenuineIntel Family 6 Model 85 Stepping 4
15:37:08: CPUs: 12
15:37:08: Memory: 31.05GiB
15:37:08: Free Memory: 28.71GiB
15:37:08: Threads: POSIX_THREADS
15:37:08: OS Version: 5.3
15:37:08: Has Battery: false
15:37:08: On Battery: false
15:37:08: UTC Offset: -4
15:37:08: PID: 5771
15:37:08: CWD: /var/lib/fahclient
15:37:08: OS: Linux 5.3.0-42-generic x86_64
15:37:08: OS Arch: AMD64
15:37:08: GPUs: 1
15:37:08: GPU 0: Bus:101 Slot:0 Func:0 NVIDIA:7 GP106GL [Quadro P2000]
15:37:08: [MED-XN71] 3935
15:37:08: CUDA Device 0: Platform:0 Device:0 Bus:101 Slot:0 Compute:6.1 Driver:10.2
15:37:08:OpenCL Device 0: Platform:0 Device:0 Bus:101 Slot:0 Compute:1.2 Driver:440.33
15:37:08:***********************************************************************
I'm running fine with CPU, but no GPU was detected so I followed steps to install opencl_dev and change 'haveGPU=true' each followed by reboots. Still don't see a GPU slot showing, and if I try to add one by changing the -1 to a 0, it doesn't error, it just adds another CPU slot with half my cores.
That's what I thought - but I don't see a GPU slot. I have this also running on a Windows machine and it shows both CPU and GPU slots - on this Ubuntu box, I only see CPU. Maybe I'm trying to add the GPU slot in the wrong way? I've just been going to 'configure->slots->add' and changing GPU-index from -1 to 0. After I hit 'ok' I end up with 2 cpu slots - the 0 that was there and a new -1 cpu slot. Each has half the cores. Clearly I'm doing something incorrect.
Solved my own foolishness. It wasn't obvious to me that I had to select the radio button next to GPU - I had thought that was for the advanced settings further down.