Gamma-aminobutyric acid (GABA) is an important neurotransmitter that promotes sleep and reduces anxiety, but its natural synthesis in the body is insufficient, necessitating dietary intake. This study utilized a combination of germination, the addition of active barley powder, and fermentation to enhance GABA content in an enzymatically hydrolyzed highland barley beverage. The samples were divided into five groups: highland barley (HB), germinated highland barley (GB), highland barley supplemented with another high-glutamic-acid decarboxylase-active highland barley powder TB13 (BT), germinated barley supplemented with TB13 (GBT), and germinated barley supplemented with TB13 followed by fermentation (GBTF). The results indicated that all the pretreatments significantly elevated GABA levels, with the GBT sample showing the highest GABA content, which was 2.4 times that of the HB sample. Germination had minimal impact on the taste and aroma of the beverage, while the addition of TB13 active barley powder caused only slight changes to the aroma. The GABA content in the GBTF sample was 2.2 times higher than in the HB sample, and the GBTF sample also exhibited the highest total phenolic content, demonstrating the strongest antioxidant and free-radical scavenging abilities. Furthermore, the GBTF treatment increased acidity, reduced bitterness, and significantly altered the flavor profile of the barley beverage, enhancing its overall quality and consumer appeal as a GABA-rich functional drink.