For some Mac OS X versions it might be necessary to install additional CUDA drivers pack to enable NVIDIA cards utilization. Drivers should be downloaded and installed from the following web-page:
After that it is recommended to reboot the system. Then re-open Agisoft Metashape Preferences window and check on your NVIDIA cards in the GPU tab.
For some latest Mac OS X versions CUDA is not supported, instead Agisoft Metashape will use OpenCL automatically for GPU-based processing.