Anytime you use the default macOS graphics driver, CUDA will NOT be enabled. You MUST use the NVIDIA Web Driver for CUDA on Mac to activate. CUDA must also be installed separately. That is directly from NVIDIA support almost a year ago when asking about the driver and CUDA performance on a GTX 680 (Mac Version) and on MBP with GeForce GT 750M.
What macOS version & build number are you using? Latest versions:
macOS 10.13.6 (17G65)
CUDA Driver Version: 396.148
GPU Driver Version: 387.10.10.15.15.108
Unless all of those versions are being used, there is a conflict somewhere.
I have no firm idea if the 2013 iMac has
only an NVIDIA GPU or if it is a hybrid/dual GPU (like the late 2013 Mac Book Pro) that contains both an NVIDIA GPU and an Intel. The late 2013 MBP has a hybrid/dual GPU with an Intel Iris Pro and GeForce GT 750M. To exclusively use the NVIDIA GPU, you need to select it in System Preferences > Energy Saver and disable automatic graphics switching.
This is what it looks like on the MBP:
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Just an FYI, this is a Mac Pro forum. You MIGHT be better off posting this in an iMac forum for more help.
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If you are POSITIVE the EFI GPU is installed correctly, with the correct power cable, seated correctly, and with only 1 monitor connected, then you should try an NVRAM reset first. You can see the instructions in the installer:
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