Then, in late April, the cancer returned a third time. Gavin's father, Chris, was told that his son had a 10-percent chance to live.
"The surgery risk at this point," Chris Rupp said, shaking his head, "you really can't do much."
Throughout the process, baseball was one of the things that kept Gavin going. A shortstop on his traveling team, he used the sport as an escape from hours of chemotherapy and radiation. His favorite team? The Washington Nationals.
And his favorite player?
"One of Gavin's wishes was to meet Bryce Harper," Chris Rupp said before Friday's game against the Padres, "and today he got to do it."
Harper spent nearly an hour with the Rupp family before the game, talking with Gavin, his parents, his brother, Ian, and his sister, Abby. The conversation was casual.
"It was just like you were sitting next to somebody on a plane and you start talking," Chris Rupp said.
They touched on everything from Harper's favorite ballpark to his pregame routine.
Harper removed his hat, signed it and put it backward on Gavin's head. He gave the 13-year-old one of his mitts and one of his bats. Then, the slugger who has signed countless autographs over the course of his career asked for a signature from Gavin and handed him a brand-new baseball. Gavin signed it.
"I'll put this in my locker," Harper said, smiling.
"With a guy who's 20 years old, to take that much time," Chris Rupp said, pausing to collect himself, "to take that much time ... if I was 20 years old, I wouldn't have the maturity to do what he just did."
For the past two years, the Nationals have partnered with Kyle's Kamp and the Children's National Medical Center to host a children's baseball tournament at Nationals Park. Hundreds of little league teams from up and down the East Coast raised money to benefit cancer research. The six teams that raised the most money played against each other at the stadium.
Gavin's team played at Nationals Park last year as the tournament raised more than $250,000. This year, it raised nearly $370,000. Chris Rupp has worked with Rob Hahne, the founder of Kyle's Kamp, to make the tournament an annual success.
As Gavin's cancer has worsened, it has started to affect his daily life. He is now in hospice care and has lost the ability to control the muscles in his face. So despite how inexplicably excited he was to see Harper, his favorite player, Gavin's face never showed it.
An hour before first pitch, Harper returned to the clubhouse to prepare for the game. Hitting coach Rick Eckstein and reliever Craig Stammen stopped by to say hello to Gavin, who quietly sipped blue Gatorade through a straw.
Forty-five minutes later, Harper returned from the clubhouse and trotted behind home plate to catch the evening's ceremonial first pitch. The pitcher? Gavin.
"When we had to tell Gavin there were no more options, he's going to die, that was hard," Chris Rupp said. "But take that day and follow it up with a day like today? It's the world."
The crowd at Nationals Park cheered as Gavin's first pitch skipped into Harper's glove. Gavin wanted to smile, but he couldn't show it. He met his favorite player in the middle of the infield for a hug and a photograph.
"There's only so much you can do as a parent," Chris Rupp said. "You can't put a price tag on this. You just can't do it."