exporting private key created with GPG to PGP

[INFORMAT]

I advised someone to use WinPT/GPG, he used it for some time, but 
finally decided to get back to PGP (version 6.0.2).
I found some incompatibility issues then which are making me doubt if it 
is such a good idea to swith to GPG anyway (although I'm strongly in 
favor for GPG) .

Export and import worked fine but:
1. First I got an error that the passphrase was not correct when I 
wanted to decrypt with PGP a document I had encrypted with GPG.
2. I did the following then: gpg --simple-sk-checksum --edit-key <KeyID> 
(changing the passphrase afterwards) and export the key then.
3. Messages encrypted with this key in GPG: PGP accepted the passphrase 
now but gave then an error "encryptes session key is bad".

What can I do to make this work (so that I'm sure someone can always go 
back to PGP if in the future there should be a problem with GPG).

Marc