Gunton, who had conducted "virtual surveillance" of targets, including a Justin Bieber concert at the Principality Stadium, was detained at his home in the Llantrisant area of south Wales on June 30 past year.
Appearing at Birmingham Crown Court previous year, he was convicted of preparing terrorist acts and propagating extremist material.
The judge told the teenager: "I sentence you on the basis that at the time of your arrest you were within hours of committing an act of atrocity on the streets of Cardiff".
But what led him to take such action?
Lloyd Gunton (dob 04/04/2000) is from Mid Glamorgan, Wales.
One post read: "May Allah bring terrorism to Cardiff on 30th June".
What did police find?The ISIS magazine, Rumiyah, promoted the tactic previous year, the premise being that those everyday items are available to most would-be terrorists.
Using Google Maps he also carried out "virtual surveillance" of a Justin Bieber concert at Principality Stadium, recalling the horror of the attack on an Ariana grande concert in Manchester that left dead.
Matthew Brook, prosecuting, told the court: "Why would he have written such a letter?"
One post alluded directly to a terror attack.
What did police do next?"The attack on Cardiff will be deadly". More news: Auto sales fall for 11th consecutive month
A post from Lloyd Gunton's Instagram account.
He also accepted he had put the hammer and knife in his bag, but claimed he had not meant to attack anyone.
At his trial the CPS was able to show from the internet history that Gunton's interest in violent extremism went back to the summer of 2016.
In the weeks before he was detained, his online searches included, "IS vehicle ramming", "how to hijack a truck" and "17-year-old Jihad", while his mobile phone contained images of the truck attacks in Nice and Berlin. "It is clear that Cardiff was your target".
Lloyd Gunton was arrested he researched security measures as part of a plan to carry out a vehicular attack at a Justin Bieber concert, The BBC reported.
Police also recovered a picture of the boy holding a gun whilst raising one finger towards the sky - a gesture often used by people associated with IS. "There will be more attacks in the future".
The judge told Gunton: "You were committed to carrying out the attack throughout a protracted period, a commitment best demonstrated by your martyrdom letter and open support for Isis on Instagram".
He said he had "always had an anti-terrorism view" and said he had searched for the material out of "gruesome curiosity".
The boy was also convicted of two counts of encouraging terrorism by posting extremist material on Instagram, and two charges of possessing Isis propaganda magazines.
He described himself as a "soldier of the Islamic state" in a "martyrdom letter" proclaiming that there would be more attacks, according to The Guardian.