The problem with PGP is that while it keeps the content of your mail exchange secret, it does not protect the information that two persons exchanged emails. So if the investigators have a lead to person A that sells drugs and see that he's communicating with person B that accepts payments, the might just guess what relationship the two have.
Right, that's why you use Tor. PGP provides privacy and accountability. Tor provides anonymity.
Just remember you have to trust the security of the receiving party as well. Make sure not to reveal yourself with the contents of your emails since the other party might not be as disciplined with their tor and pgp use as you are.
