From GnuPG site…
A severe problem with ElGamal sign+encrypt keys has been found. This leads to a full compromise of the private key. Fortunately those keys are not in wide use and only creatable using special options.
Phong Nguyen identified a severe bug in the way GnuPG creates and uses ElGamal keys for signing. This is a significant security failure which can lead to a compromise of almost all ElGamal keys used for signing. Note that this is a real world vulnerability which will reveal your private key within a few seconds.
Please take immediate action and revoke your ElGamal signing keys. Furthermore you should take whatever measures necessary to limit the damage done for signed or encrypted documents using that key
More information here.