You have enabled checking of packages via GPG keys. This is a good thing.
However, you do not have any GPG public keys installed. You need to download
the keys for packages you wish to install and install them.
You can do that by running the command:
    rpm --import public.gpg.key


Alternatively you can specify the url to the key you would like to use
for a repository in the 'gpgkey' option in a repository section and yum
will install it for you.

For more information contact your distribution or package provider.