This is so "I can live in it when I'm a gorilla", he says.
Garrett's family said that during the nine months of hell, he never lost his sense of humor and loved to tease doctors and nurses.
He said he wanted fireworks at his funeral and ended off the letter with, "See ya later suckas! - The Great Garrett Underpants".
The obit was compiled by his parents, Emilie and Ryan Matthias, who began asking Garrett questions about adult topics such as funeral and burial preferences when they learned from doctors last month that his cancer was terminal.
The words he spoke to them now form his touching obituary. He says that when he grows up, he wants to be a professional boxer. My address is: I am a Bulldog!
He goes on to declare that when he dies, he is "going to be a gorilla and throw poo at Daddy!".
Garrett also lays out his funeral requests, including being burned "like when Thor's mommy died" and being "made into a tree so I can live in it when I'm a gorilla".
The result was the "Celebration of Life" in honor of the 5-year-old, due to take place Saturday in Van Meter, Iowa, complete with "5 bouncy houses (because I'm 5), Batman, and snow cones".
Garrett, of Van Meter, Iowa, was diagnosed with alveolar fusion negative rhabdomyosarcoma in September, according to the Des Moines Register.
It eventually spread to other parts of his body.
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In the obituary, Emilie and Ryan wrote that they intend to fight for a cure for cancer until "no other kids are robbed of their childhood".
Things I hate: trousers, dirty stupid cancer, when they access my port, needles and the monkey nose that smells like cherry farts.
He also says he dislikes trousers.
Garrett then lists some of his favourite things, including superheroes Batman and Thor, his bunny, playing with his sister and Lego.
Other "favourite people" named by the little boy included his "grandparents with the new house", "grandparents with the camper", aunts and uncles, cousins and "Stinky Uncle Andy".
His responses, his mother said Friday, made up the obituary.
"That's him speaking. Those are his words verbatim", he said.
"I'd say things like, 'When I die, I want to turn into a star, '" Emilie Matthias told the Register. "Garrett was a very unique individual". Over the past few months, Garrett and his parents had some frank discussions about death, and at one point Garrett noted, "Why are funerals so sad? From whoopy cushions and sneaking clothes pins on their clothes to "hazing" the interns and new staff doctors, he was forever a prankster", they added, before signing off with this message.
He never lost his sense of humour.
Five-year-old Garrett Matthias endured months of brutal treatment for a rare cancer that ultimately took his life July 6. His parents say they're going to do just that, "once [they] figure out how the hell to get his ashes made into a tree".