The Plymouth College pupil claimed gold in the 13/14 year-old boys’ 50m backstroke at Ponds Forge. He clocked 29.16 seconds.
Gdaniec’s gold followed on from his Leander team-mate Ben Fannon’s on the opening day of the meet.
Mount Kelly also continued their good form at the national competition on Thursday.
Katelyn Bergin secured silver in the women’s 200m IM with a time of 2:22.84, while Samuel Pease took bronze in the men’s 50m backstroke with a 27.09 second swim.
There were also finals for fellow Mount Kelly swimmers Maxwell George, Benji Gillingham, Sophie Main, Jade Blake, Sara Mazzeo, Lewis Smart and Jacob Newcombe, as well as Leander’s Batuhan Paker, Ethan Rayment and Finlay Washburn.