Gainer turned professional in January 2020 after the LET Q-School, and joined the
LET Access Series. In 2020, she recorded four top-20 finishes in five starts including a runner-up finish at the
Czech Ladies Challenge in Prague. She ended the year in second place on the Order of Merit behind
Tiia Koivisto, earning promotion to the
Ladies European Tour (LET). She was also crowned LETAS Rookie of the Year. In 2023, Gainer finished 3rd at the
La Sella Open and lost a playoff at the
Amundi German Masters to
Kristýna Napoleaová to finish the season 16th in the Order of Merit. She captured her maiden LET title at the 2025
Lalla Meryem Cup by birdying the first hole in a playoff against
Diksha Dagar. ==Amateur wins==