He made the astonishing claim at the Paraforce UK paranormal convention in Witham, Essex.
Mr Brouwer, who was visiting from the Netherlands, told the stunned audience that in the year before his possession, a terrible series of events happened.
He said: “I had a car crash the same year aged 20, I lost my girlfriend in the same year and a friend of mine was murdered.
“I thought ‘why am I getting so much bulls*** in my life.'”
He said he got himself a new apartment, but after just two months of being there he “saw a web coming down with a big spider in it”.
He said: “I put the lights on and there was nothing there.
“I came home two weeks later and could not breathe anymore.
“A neighbour was knocking on the door saying Gert, Gert, Gert… she wanted to help me, but I couldn’t breathe anymore.
“I had scratches, my brain was not working anymore and I couldn’t breathe, so I went to see a shrink.”
Mr Brouwer began writing two blogs – one called “Me” and the other “He”.
He said: “I was asked why do you call it he, and I said I felt ‘He’ was a demon. I hope no one here will get caught with something like that.”
Mr Brouwer said after about two and a half months, he decided he could defeat the creature.
He said: “I changed my stance and had the psychology ‘I want to beat you… I want to win this battle’.
“I needed to reschedule my thoughts because I was mixed up with a demon.”
He decided to train as an exorcist to be able to fend off the hellish spirit.
He said he now knows just how rare demonic possession cases are.
Mr Brouwer, who was born in the Netherlands, and now lives there in Breda, said: “In 95 percent of possession cases there is no demon involved, it is just the emotions of an entity.”
He told the convention, he had paranormal experiences from as young as 11, when he also saw a psychiatrist.
Mr Brouwer said: “Aged 11 I would see things, pictures, and told my shrink what would happen in the future.”
He went on to found Paranormal Society World and GJB Media and describes himself as a demonologist, medium and exorcist.
The website of the society says he has won 14 awards for his paranormal work and appeared on TV.
His profile on the website states: “From a young age, Gert could communicate with the immortal.
“He uses all his senses in the field. At his 21st year, he was confronted with a demon in his apartment. This was also his first negative experience in the paranormal.
“This made the road open for Gert as an exorcist and the occult.”
There is no scientific evidence that possession is real, but many religions carry out exorcisms to rid people of evil spirits.
The oldest references to demonic possession are from the Sumerians, from around 4,000BC.
They believed all diseases of body and mind were caused by “sickness demons” called gidim or gid-dim.