John Green has written several novels, including:
* Looking for Alaska (2005)
* An Abundance of Katherines (2006)
* Paper Towns (2008)
* Will Grayson, Will Grayson (2010, co-written with David Levithan)
* The Fault in Our Stars (2012)
* Turtles All the Way Down (2017)
* The Anthropocene Reviewed (2021)
He is also a popular YouTuber and podcast host.
John G Franzen, on the other hand, is a different author. He is known for his literary fiction and has written several critically acclaimed novels including:
* The Corrections (2001)
* Freedom (2010)
* Purity (2015)
* The Kraus Project (2018)
* Crossroads (2021)
You might be thinking of one of these authors.