Oh you already have it installed. I thought when you said 'Graphics Driver selected' you were talking about the ubuntu settings menu.
In that case, search for 'Nvidia X Server Settings' (called nvidia-settings in terminal), search for something called PRIME (should be called 'PRIME profiles'). There you can set your preferred gpu.
If all else fails, and you are 100% sure nvidia drivers are installed correctly, you could try
__NV_PRIME_RENDER_OFFLOAD=1 ./Zero-K_linux64
in the zero-k directory (assuming non-steam, if on steam you'll have to figure out how to add that argument to the steam command line thing. Shouldn't be too complicated).
I'll admit I don't know much about PRIME, but the above is worth a shot.
https://download.nvidia.com/XFree86/Linux-x86_64/525.89.02/README/primerenderoffload.html