Former Sydney FC left-back Gersbach came on in the 82nd minute as Rosenborg claimed their second successive Norwegian double with a 4-0 thrashing of Kongsvinger in the Cup final.
The 19-year-old replaced hat-trick hero Paal Andre Helland as he entered the fray in front of a near-capacity crowd of 26,000 at Oslo's Ullevaal Stadion.
The result completes a whirlwind year for Gersbach, who made the move to Norway in January this year, before debuting for Australia five months later.
Meanwhile, fellow Socceroos defender Sainsbury could also be set for some silverware after his club claimed a crucial away goal in the first-leg of the Chinese FA Cup final against powerhouse Guangzhou Evergrande.
The 1-1 result, thanks to Alex Teixeira's second-half equaliser, sets things up nicely for Jiangsu who host the second-leg on Sunday night (AEDT).
And a delighted Sainsbury, who played the full 90 minutes, took to Twitter ahead of the return home leg.