On 26 March 2011, at the New Zealand National Championships, Gill achieved 21.34m with the 6 kg shot, at the time ranking him 4th on the all-time IAAF junior list for shot put.
On 23 April 2011, at 16 years of age, Gill threw 20.01m with the senior shot (7.26 kg). Gill's throw broke the 44-year-old New Zealand national record set by Les Mills in 1967. This performance surpassed the standard for the 2011 Athletics World Championships, however the rules for that competition exclude anyone of Jacko's age competing in any throwing events. An appeal to the IAAF for an exception to be made to this regulation to allow Gill to compete was rejected. The performance also surpassed the B standard for the 2012 Olympics however not only was it outside the qualifying period (which started on 1 May) it was also less than the qualification standard (20.30m) subsequently set by Athletics New Zealand. However, he did go on to qualify for the 2012 Olympics on 5 December 2011 by throwing 20.38m. On the same occasion he threw 22.31 with the 6 kg shot to become the second furthest junior (under-20) thrower ever with that weight. Gill has been reported as the youngest and one of the lightest athletes to throw over 20m.
On 7 July 2011 at the 2011 World Youth Championships in Athletics in Lille Métropole, France, Gill won the gold medal by 4m with a World Youth Best Performance of 24.35m, which he subsequently improved later that year with a throw of 24.45m in Auckland on 12 December.