When a recipient using GPG attempts to encrypt to your PGP key, an error similar to the following is displayed:
GPG: SKIPPED: UNUSABLE PUBLIC KEY
GPG: TEST.TXT: SIGN+ENCRYPT FAILED: UNUSABLE PUBLIC KEY
Although GPG is generally compatible with PGP and vice versa, it is not necessary for GPG to comply with the PGP standard and therefore some PGP functions may not be compatible with the GPG software and vice versa.
The solution to this particular issue is to create a new PGP RSA key instead of DH/DSS key and then distribute the new key to the GPG client to use for encryption. This is not a PGP issue, but rather the GPG software not complying with a PGP standard.
When creating a PGP key with PGP Desktop 9.x, by default the key is created as a RSA key. A PGP RSA key displays as a blue keypair in PGP Desktop while a DH/DSS key displays an orange key.
For more information on how to create a new PGP key, click here