You may have GPG error (NO PUBKEY) related to nvidia’s cuda when you apt-get update like:
GPG error: http://developer.download.nvidia.com Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY F60F4B3D7FA2AF80
The solution is to add public key to your system. In my case, the following command first downloads an appropriate pubkey file and then add the downloaded key to the system.
wget -qO – http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1404/x86_64/7fa2af80.pub | sudo apt-key add –