Grgić won the gold medal both in the 800 m freestyle and the 1500 m freestyle event at the 2019 FINA World Junior Swimming Championships in Budapest, Hungary. With the time of 14:46.09 in World Junior Championship he is the fastest 16-year-old 1500m freestyle swimmer in history. He captured two gold medals in the 400 m freestyle and the 1500 m freestyle event at the 2019 European Youth Summer Olympic Festival (EYOF) in Baku, Azerbaijan.