The government does not admit that it exists, but is researching the possibility.
Blood tests on sick US veterans show that many produced high levels of antibodies against a substance called squalene.
Those veterans who had not complained of illness since the 1991 war did not produce antibodies.
This is considered normal, as the body does not contain enough squalene to provoke an immune response.
The researchers suggest that GWS is caused by the body turning its immune system against its own natural supply of squalene.
The findings are reported in New Scientist magazine.
Controversial illness
GWS is a mysterious illness that involves memory loss, thyroid disorders, allergies, fatigue, rashes and persistent pain.
It affects 100,000 veterans of the war.
The government does not admit that it exists, but is researching the possibility.
Some campaigners say the syndrome is caused by vaccines given to troops, while others believe uranium used in weapons is the cause.
In January, researchers at King's College, London, found that exposure to plague and anthrax vaccines was the factor that correlated most strongly with GWS in British victims.
But the US Defense Department denies squalene was used in its vaccines.
Increased antibody levels
The latest research, however, suggests the sick veterans were exposed to high levels of sqalene.
It was conducted by Dr Bob Garry, a virologist at Tulane University in New Orleans.
He tested 400 veterans for antibodies to squalene and found that 95% of those who claimed to have GWS had high levels.
In normal circumstances, squalene can be released into the blood by physical injuries where it boosts the immune system's response to foreign antigens.
Dr Garry also tested two volunteers who had received experimental herpes vaccines containing squalene in trials run by the US National Institutes of Health.
Both had high levels of squalene antibodies and symptoms similar to GWS, suggesting GWS could be caused by the body turning against its own squalene.