The Brazilain pathologist, i forget her name, is an old doc, very experienced. She ruled his death a suicide, so Amanda had to be released charges dropped.
Some shady private investigators that Gatti's family (who was suing his wife over his estate at the time) hired claimed that they had proof he was murdered. The coroners who reviewed the case in both Brazil and Canada ruled his death a suicide.
The surveillance tape showed that no one besides Gatti or his wife entered that place that night. So either Gatti killed himself or his wife did it herself.