Very old versions of PGP (2.x) used patented encryption algorithms (IDEA/RSA) to encrypt secret keys. As a philosophical matter, the GnuPG does not contain code for patented algorithms. The consequence is that gpg cannot decrypt and use old pgp secret keys. That means that gpg won't be able to do anything that would require your passphrase: decrypting a message sent to you, or signing a message, for example. However, public keys are not encrypted so gpg can still be used to encrypt messages to users with old keys (although they'll have to use something other than gpg to read the messages).
Attempting to use gpg with an old key will produce an error message like the following:% gpg -s something.txt
It's possible to add IDEA support back in by obtaining a patch and recompiling gpg. See the gnupg web page for details.