[15] Four months later, in March 2018 and when she was 15 years old, Walsh became the youngest female American swimmer to swim the 50 yard freestyle race in less than 22.00 seconds with a time of 21.85 seconds that also broke the former National Age Group records in the event for the girls 15–16 age group set by both Simone Manuel and Kate Douglass at 22.04 on different dates.

[3][18] Competing as part of the Nashville Aquatic Club at the 2019 Speedo Southern Premier Meet in Knoxville, Walsh broke the girls 15–16 age group National Age Group record of 47.73 seconds in the 100 yard freestyle set in 2013 by Simone Manuel with a time of 47.49 seconds.

[19] In May 2019 SwimSwam ranked Walsh as the number one NCAA recruit from all swimmers across the United States in the high school graduating class of 2021.

[33] At the 2020 US Olympic Trials in Omaha, Nebraska, which were postponed to 2021 due to the COVID-19 pandemic, Walsh started competing on the first day of the second wave of the competition, swimming a time of 27.02 seconds in the 50 meter butterfly en route to her final mark of 58.58 in the prelims heats of the 100 meter butterfly, which ranked her eighth heading into the semifinals.

[34] In the semifinals of the 100 meter butterfly, Walsh swam a time of 58.46 seconds, ranked twelfth overall, and did not advance to the final of the event.

[2][10] In January 2022, Walsh split a 23.04 for the lead-off leg of the 4×50 medley relay in a dual meet against North Carolina State University, becoming the fastest female American swimmer in the 50 yard backstroke.

[47] In the final of the 4×50 yard freestyle relay in the evening, she split a 20.58 on the fourth leg to help achieve a first-place finish in an American record time of 1:24.47.

[48][49][50] Walsh swam a 21.04 in the final of the 50 yard freestyle to become the fourth-fastest woman ever to swim the event in the NCAA, behind Abbey Weitzeil, Kate Douglass, and Erika Brown, and place second.

[55] She followed up her individual performance with a split of 49.71 on the backstroke leg of the 4×100 yard medley relay, helping set new American and US Open records in the event at 3:22.34.

[60] Finishing off the meet, she anchored the 4×100 yard freestyle relay with a 46.35, helping win in an ACC record time of 3:08.22.

[69] In the final, she won the NCAA title in the event, set a new pool record, and ranked as the fourth-fastest all-time in the 100 yard freestyle with a new personal best time of 46.05 seconds.

[71][72] Walsh ranked 22nd in the preliminary heats of the 100 meter freestyle on day one of the 2022 US International Team Trials in Greensboro, North Carolina, in April, not advancing to the final with her time of 55.57 seconds.

[73] The following day, she tied for fourth-rank in the prelims heats of the 50 meter butterfly with a personal best time of 25.98 seconds, advancing to the evening final.

[75] On day three, she qualified for the final of the 50 meter backstroke ranking eighth in the prelims heats with a personal best time of 28.26 seconds.

[86] The first night of the 2023 Atlantic Coast Conference Championships in Greensboro, North Carolina, Walsh led-off with a personal best time of 22.65 seconds for the backstroke portion of the 4×50 yard medley relay to help win the conference title in an American, US Open, and NCAA record time of 1:31.73.

[87][88][89][90][91] In her first final of the evening session the following day, she split a 20.48 for the second leg of the 4×50 yard freestyle relay, helping set new American, US Open, and NCAA records with a time of 1:23.87.

[92][99] In the final of the 4×100 yard medley relay the following day, she contributed to winning the conference title with a 3:21.80, swimming the backstroke leg in 49.25 and helping set new US Open and NCAA records in the event.

[111] She attained a sub-46 second time (45.85) on the anchor leg of the 4×100 yard freestyle relay to conclude the meet with another NCAA title in an American, US Open, and NCAA record time of 3:05.84 with relay teammates Kate Douglass, her elder sister, Alex Walsh, and Maxine Parker.

[115] At the 2024 NCAA Championships, Walsh swept all seven of her events, contributing strongly to the Virginia Cavaliers' victory, their fourth consecutive one.

[116] On the second day, Walsh twice lowered her NCAA and American records in the 50 yard freestyle, swimming a 20.41 in the preliminary heats before blasting a 20.37 in the finals to take first.

She additionally split a 20.23 on the second leg of the 4x50yd freestyle relay, culminating in Virginia winning with a time of 1:24.05, marking a meet and pool record.

[116] She then swam a 48.26 in the 100 yard backstroke to open Virginia's 4x100yd medley relay, which won in a 3:21.01, breaking the NCAA and American records.

On the meet's last day, Walsh won the 100 yard freestyle in a 44.83, marking the first time any woman broke 45 seconds in the event.

[124] Gretchen and her sister Alex were the first NCAA athletes (and set of siblings) to launch an apparel line with a major retailer after the passage of the NIL.