The forward scored 13 points in just 18 minutes game time as Serbia were beaten 76-68 - though Damian Jennings' side left it late in a nervy performance.
Gandy was joint top scorer with Azania Stewart, while Jo Leedham - back in national colours for the first time since last year's Olympics - also impressed.
However, Great Britain - eight places ahead of Serbia in the current FIBA rankings - still entered the final period trailing by seven points.
Ella Clark, in her first international appearance, cut the deficit to one with a long range score and Rachel Vanderwal's three-pointer, with just over three minutes remaining, not only gave Great Britain the lead but the belief they could go on and win.
Leedham steadied nerves from the line and Stef Collins eased the pressure on the scoreline late on as Serbia's resolve was broken.
However, Great Britain will need to improve ahead of Sunday's match with world number 16 side Latvia and their clash with other group C rivals, France.