Gillaspie spent the 2019 season with the Wisconsin Timber Rattlers of the Single-AMidwest League, posting a 3–7 win–loss record with a 3.96 ERA over 31 games (16 starts), striking out 92 batters over 109 innings.[6] Gillaspie was named a Midwest League all-star with the Timber Rattlers.[7] On September 11, 2019, Gillaspie was released by the Brewers organization. He did not play professionally in 2020 due to cancellation of the Minor League Baseball season due to the COVID-19 pandemic.[8]
On November 19, 2021, the Orioles selected Gillaspie's contract and added him to their 40-man roster to protect him from the Rule 5 draft.[11] He returned to Bowie to open the 2022 season.[12] After six appearances with Bowie in 2022, he was promoted to the Norfolk Tides of the Triple-AInternational League.[13] On May 17, 2022, the Orioles promoted Gillaspie to the major leagues for the first time. He made his MLB debut that night versus the New York Yankees at Oriole Park at Camden Yards, throwing two scoreless innings in relief.[14] On October 2, Gillaspie threw a scoreless 6th inning against the New York Yankees at Yankee Stadium to record his first major-league victory in a 3–1 Orioles win.[15] Gillaspie finished the season with a 1–0 record with a 3.12 ERA in 17+1⁄3 innings pitched in 17 appearances.
In 2023, Gillaspie made 11 appearances for Baltimore, but struggled to a 6.00 ERA with eight strikeouts in nine innings of work. On September 2, 2023, he was designated for assignment following the Orioles' waiver claim of Jorge López.[16]
San Diego Padres
On September 4, 2023, Gillaspie was claimed off waivers by the Boston Red Sox[17] and optioned to Boston's Triple-A affiliate, the Worcester Red Sox.[18] On November 15, 2023, Gillaspie was claimed off waivers by the San Diego Padres.[19] He was optioned to the Triple–A El Paso Chihuahuas to begin the 2024 season.[20] On April 17, Gillaspie's contract was selected by the Padres.[21] Two days later, Gillaspie was brought in to relieve in the 9th inning for the Padres against the Toronto Blue Jays with a runner on third base and one out, where he struck out both Bo Bichette and Vladimir Guerrero Jr.[22]