PGP5/gpg armoring

[Fabio Coatti]

Well, I've been able to import pgp5 secret keys in my gpg keyring (I've
removed the pgp passphrase, then exported the key from secring.skr and
reimported the same in gpg secret keyring and finally replaced the
passprhase) and gpg is now able to decrypt messages encrypted by pgp5 using
the pubkey.
But If I try to encrypt a message using gpg, I get a 
Unusable pubkey algorithm
What does it means?
Maybe the pgp pubkey have a invalid lenght?

pub  1024D/40AC5709 1997-09-03 Fabio Coatti <cova at felix.unife.it>
sub  4096g/96E96B20 1997-09-03

And I'm unable to check signatures (with pgp5) made with gpg 
using the same key. Has anyone tried something liker this? Any success? 
Thanks for any help...

-- 
Fabio Coatti
2:332/409.414                   Fidonet
cova at felix.unife.it             Internet
http://felix.unife.it/~cova     Home page
Old SysOps never die... they simply forget their password.