While updating a Debian based system, you may encounter an error as follows:
This is a feature of the apt-get package manager that checks the authenticity of servers while updating Debian. It just means that the system is unsure if the repository it listed is safe to receive updates from.
To fix this problem (provided you are sure that the repository is safe), all you need to do is execute the following commands with the pubkey you received in the error:
gpg --keyserver pgpkeys.mit.edu --recv-key 2836CB0A8AC93F7A
gpg -a --export 2836CB0A8AC93F7A | sudo apt-key add -
From now on Apt-Get will recognize that repository and not give you a GPG error.