Gardner's 66 runs from 38 balls at Arundel Castle helped Australia to an 85-run win over the Proteas, who eliminated them from the last edition of the tournament in a semi-final boilover.
Australia, the world's top-ranked side, enter this month's World Cup with great expectations on their shoulders, having also failed to defend their 50-over title in 2025.
Gardner's fellow allrounder Georgia Wareham (52 runs off 29 balls) chipped in with a half-century of her own to help Australia to 6-212 on Thursday (Friday AEST), when South Africa finished at 6-127 in reply.
South Africa opener Laura Wolvaardt (47) retired just shy of her half-century, with Ellyse Perry (1-8), Annabel Sutherland (1-12) and new captain Sophie Molineux (1-16) helping rattle through her teammates.
Gardner had not made it out of single digits with the bat in either of Australia's first two friendlies against the Proteas, but was relieved to find form.
"I haven't been hitting the ball that well in the nets so to actually be able to do it in more a game scenario is certainly pleasing and gives you a lot of confidence," she said.
"The nets only kind of say one thing and then when you actually get out in the heat of the battle, that kind of dictates where your game's at a little bit more."
Australia now travel to Cardiff and will face England and West Indies on June 8 and 10 for official warm-up matches, before the tournament proper begins against South Africa on June 13.
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