A couple of options that I can think of:
1.) Buy a Cray and set it to work guessing the passphrase. Then go on vacation while it guesses the password.
2.) Drag the ex-admin into court to get the passphrase out of him.
If you think about it, the passphrase, and the passwords, are company property. Bearing that in mind you probably have a good legal argument for getting the passphrase and/or passwords out of the ex-admin.
Sorry for not being too constructive. The problem you have, though, is that PGP uses the IDEA algorithm to encrypt data. Without the passphrase it takes a long, long time to get the data back. I wasn't kidding about the Cray. Even with something that big, and a good cracking tool, it is estimated that you would need a good couple of weeks to crack a PGP file that had a good passphrase and a large key length. If you were lucky.
Good luck in getting everything back.