Garbers attended Corona del Mar High School in Newport Beach, California. He was the full-time starting quarterback in his 2015 junior and 2016 senior campaigns, earning Daily PilotDream TeamPlayer of the Year and first-team All-Pacific Coast League honors in both years, while he was also his conference MVP as a senior. Over his high school career, Garbers had a combined 589-of-854 passes for 7,970 yards with 90 touchdowns and 10 interceptions for a 129.1 quarterback rating. Garbers was listed as four-star recruit, ranked No. 12 pro-style quarterback and No. 315 overall prospect in the country for the 2017 class.
In 2018, Garbers shared Cal's starting quarterback position with Brandon McIlwain in a two-quarterback system, though Garbers started the majority of the season. In his first career start, Garbers completed 18 of 28 passes for 171 yards and two touchdowns against BYU. The Bears finished the 2018 season with a 7–6 record, including a loss at the Cheez-It Bowl against the TCU Horned Frogs. Struggling to find a rhythm on offense, the Bears' offensive efficiency ranked as the second worst among all Power Five teams. Garbers finished the season with 14 touchdowns, 10 interceptions, and 1,506 yards passing.
In 2019, Garbers was named Cal's starting quarterback. He helped lead the Bears to a 4–0 start to the season, including a 20–19 upset win against No. 14 ranked Washington, earning the Bears a No. 15 ranking, their highest since 2009. Garbers had career highs in completions, yards passing, and touchdown passes in the Ole Miss game with 23-of-35 passes for 357 yards with four touchdown throws. Garbers suffered a broken collarbone during the Arizona State game that sidelined him for the next five games, four of which were losses. He returned briefly for the USC game, but sustained a concussion close to the end of the first half. Garbers returned to lead the Bears to a 24–20 win against Stanford in the Big Game, the first Big Game win for Cal since 2009. Garbers ran for a 16-yard game-winning touchdown, earning praise from Green Bay Packers quarterback and former Cal quarterback Aaron Rodgers. This win clinched bowl eligibility for the Bears while making the Cardinal ineligible for the post-season for the first time in ten years. The Bears went to the Redbox Bowl, where they won against the Illinois Fighting Illini 35–20, their first bowl win since 2015. Garbers had a career best-tying 4 touchdowns and was named Offensive MVP for the game. The Bears finished the 2019 season with an 8–5 record, with Garbers going undefeated in the 7 games in which he started and finished.
