GPG Error on Synaptic Update

how to fix GPG error on Ubuntu Synaptic Update

I got some GPG error while updating the system:

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://fai-project.org wheezy Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2BF8D9FE074BCDE4

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D45DF2E8FC91AE7E
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4F191A5A8844C542
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FAC463C9A207A89
W: Failed to fetch http://fai-project.org/download/dists/wheezy/Release  

W: Failed to fetch http://dl.google.com/linux/deb/dists/stable/Release  Unable to find expected entry 'non-free/binary-amd64/Packages' in Release file (Wrong sources.list entry or malformed file)

W: Some index files failed to download. They have been ignored, or old ones used instead.

GPG ERROR fixed

The problem could be fixed with this command (this will add the key to the system and signatures will be verified):

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4F191A5A8844C542

Repleace red part with each error key and just run it on ubuntu terminal then do an sudo apt-get update again.