So Marshall decided to get a tattoo to match his son's scar.
"This broke my heart," Marshall said. "I told him if people wanted to stare, they could stare at both of us."
Although he got the tattoo in August 2015, Marshall entered St. Baldrick's Foundation's #BestBaldDad competition on Father's Day.
The competition showcases the solidarity between dads who shave their heads and their children who are undergoing cancer treatments that leave them bald.
Marshall's entry was chosen as the winner and led to an image of his tattoo next to his son's scar getting picked up by multiple news outlets.
Marshall can't believe the response.
"I never intended for it to be this big," he told ABC News. "It was just a friendly competition between fellow cancer dads and people showing support for their loved ones who have fought cancer."
St. Baldrick's Foundation's social media manager told BuzzFeed News Marshall "went above and beyond shaving his head in solidarity with his child and got a tattoo to build his son's self-confidence. It's a truly touching story."
Marshall told ABC News a small amount of the tumor remains in Gabriel's head, but that he is doing well and the tumor, which is monitored by MRIs every three months, has not grown.