You're supposed to sign the message with a bitcoin address, not a GPG key, though if you claimed ownership of the GPG key in a public post on your main account it should still work (I'm not sure if signatures count). Also, you need a link to the post from your main account with the address or GPG key in it; otherwise there's no way to verify the key is really yours.
For more information, see this thread (though I suspect you already have):
https://bitcointalk.org/index.php?topic=497545.0Actually theymos will accept a PGP signed message as it is just as secure as a signed bitcoin message (plus not all users will retain the private keys to every address they post)