Gregson finished 5th in the boys' 1500 metres at the 2007 World Youth Championships in Ostrava. Later that year, he set new Australian youth records over 1500 metres, 3000 metres and 5000 metres, his 3000m clocking of 8:01.26 also eclipsing Craig Mottram's Australian junior record. At the 2008 World Junior Championships in Bydgoszcz he again finished 5th over 1500 metres and also participated in the 5000 metres. In 2009, his last year as a junior, he set new Australian junior records over both 3000 and 1500 metres, with times of 7:57.45 and 3:37.24 respectively.