I have read a lot of claims regarding GAP syndrome and GAPS diet with the most surprising of them being that the proposed diet can even cure disorders such as autism and schizophrenia. For example in this link, Dr Campbell states:
...The toxicity, which is produced by the abnormal microbial mass in these patients, establishes a link between the gut and the brain. That is why I grouped these disorders under one name: the Gut and Psychology Syndrome (GAP Syndrome or GAPS). The GAPS children and adults can present with symptoms of autism, ADHD, ADD, OCD, dyslexia, dyspraxia, schizophrenia, depression, bipolar disorder, sleep disorders, allergies, asthma and eczema in any possible combination. These are the patients who fall in the gap in our medical knowledge. Any child or adult with a learning disability, neurological or psychological problems and allergies should be thoroughly examined for gut dysbiosis. Re-establishing normal gut flora and treating the digestive system of the person has to be the number one treatment for these disorders, before considering any other treatments with drugs or otherwise.
Although these claims seem unrealistic to me, I would like to find solid scientific evidence that refutes them.