The 17-year-old triumphed in a three-hole play-off over England’s Max Orrin and Slovenia’s Katja Pogacar after the trio finished on nine-over-par 255 for the 54 holes.
The tournament – inaugurated in 2001 – is an international strokeplay event for boys and girls, who are either the current holders of their under-18 national championship or have won some other major golfing event in the preceding year.
Pogacar, the Slovenian Girls Champion, held a two-shot lead overnight, and three-putted on the final green to fall back into the play-off.
The 18-year-old missed from four feet at the first before bogeying the 18th, the second play-off hole, to end her hopes of becoming only the second female winner of the event.
Gardarsson and Orrin, 18, continued the play-off, with the former prevailing after a fine up-and-down from sand at the third extra hole.
“It’s the best round I’ve ever played in bad weather. It’s been a really fun this week and I’ve enjoyed it very much,” said Gardarsson.
“Thanks to all involved for a wonderful tournament on an amazing course. It’s been a great experience and I’ve made many new friends.”
Orrin, who missed from six feet to win at the second extra hole, added: “I thought it was going to be Katja’s week, so it was disappointing for me to miss my chance to win.
“I’m frustrated, but there are so many positives for me.”