Both program use the OpenPGP format and thus it should be no problem to exchange encrypted mails from GPG to PGP.
But some things might cause trouble:
GPG does not support the IDEA algorithm and if the
PGP key prefers IDEA, GPG won’t be able to read it.
PGP does not support Blowfish and if the GPG key prefers this algorithm PGP won’t be able to read it.
But with the default settings CAST5/AES no problem should occur.