I am 99% sure that the signature does not contain email address information. So the only way you are going to find the email address(es) is by finding the key.
When gnupg tries to validate the signature it should tell you the key ID. Using the key ID you can try to retrieve the key from the keyservers with
gpg [--keyserver <server>] --recv-keys <keyid> You can try various different keyservers. Usually I try whatever the default is in my gpg installation, if that doesn't work I try "the.earth.li" and if that doesn't work I try "pgp.mit.edu". Sometimes one keyserver will have keys that others don't.
You can also try searching for the key ID using google.
Finally note that when you do find the key you should not blindly trust it. You should take appropriate steps to check it legitimately belongs to a person/entity you trust.