Refer to GPG error: http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64 - #3 by pablete
Try:
$ sudo apt-key del 7fa2af80
$ sudo apt-key adv --fetch-keys http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1604/x86_64/3bf863cc.pub
More error or question, you can search in that CUDA forum or create a topic on it. Thanks.