What you are effectively trying to connect to is an RDP session. NoMachine will connect to the physical display of the host (not an RDP session already running). Maybe in the future NoMachine for Windows will support “virtual” sessions. Right now it will connect you to the physical display of the host.
The fact is that without a display adapter, no rendering can even happen on the server. The exception is RDP, but only because it creates a virtual session – it doesn’t connect to the physical display.
However, it seems that now Google Cloud makes available instances with a GPU (at least in beta, as far as I can see). You may want to check that here: https://cloud.google.com/gpu/.