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 previous year.
The teenager, who suffers from an autism spectrum disorder, was convicted of preparing for terrorist acts after a nine-day trial at Birmingham Crown Court last November.
But what led him to take such action?
Lloyd Gunton (dob 04/04/2000) is from Mid Glamorgan, Wales.
His password for the social media account was "truck attack" authorities said.
One post read: "May Allah bring terrorism to Cardiff on 30th June".
Two copies of an IS propaganda magazine were found on his laptop.
The teen also looked up how to carry out an attack using a vehicle, and how to stab and kill someone. Both editions praised terrorists and encouraged others to commit terrorist acts.
Ordering Gunton to be detained at Her Majesty's pleasure, Judge Mark Wall QC told the former A-level student: "The police found a rucksack in your room which contained a knife, a hammer and what has been referred to as a suicide note or a martyrdom letter". The letter was written in such terms that it was obviously to be found and read after you had carried out a terrorist attack...
One post alluded directly to a terror attack.
May defends stance on post-Brexit financial services rules
The EU's main negotiator, Michel Barnier, welcomed the "clarity" May offered and her recognition that Britain faces "trade-offs". She says Britain wants a close relationship after Brexit , but not one modeled on existing trade deals with Norway or Canada.
What did police do next?"The attack on Cardiff will be deadly".
He also accepted he had put the hammer and knife in his bag, but claimed he had not meant to attack anyone.
Analysis of his internet searches showed he had an "unhealthy interest in terrorism", and had searched IS leader Abu Bakr al-Baghdadi's name eight months before his arrest in late June.
The court heard he also researched possible target sites in Cardiff, including Cardiff Castle, New Theatre, Capitol shopping centre, Cardiff Central Library and Bridgend's McArthur Glen shopping outlet.
17-year-old Lloyd Gunton from Rhondda Cynon Taff was arrested in June 2017.
"Lloyd Gunton's behaviour over many months leaves no doubt that he meant to kill and maim as many people as possible in an attack reminiscent of the incident on Westminster Bridge".
"ISIS beheading video", "How to create a terror attack" and "What does getting shot feel like".
He did admit that he was curious about ISIS, but he said that he "never thought about actually" carrying out an attack.
He said he had "always had an anti-terrorism view" and said he had searched for the material out of "gruesome curiosity".
He was charged with one count of engaging in the preparation of a terrorist act, two counts of encouraging terrorism, and two counts of possessing terrorist information.