GLA *may* help lessen cravings for alcohol and prevent liver damage, but thoses studies and only preliminary.
Hangovers are caused by excessive alcohol consumption, pure and simple, and are an immediate withdrawl symptom. You can free yourself from the symptoms with more alcohol (hair of the dog), with other drugs that have central nervous system depressing effects such as Benzodiazapines (Valium being the most effective), Opiates (like Vicodin), drugs that contain Barbituate (such as Phenobarbitol), GHB and of course time, the healer of all wounds.
Coffee can help with the headache and contains antioxidants that support liver function, and I've heard that Prickly Pear cactus extract can reduce the nasuea. But again, the best prevention is to not drink like an Indian.