In this study, broilers were selected as the research object to investigate the effects and mechanism action of dietary gallic acid (GA) supplementation on growth performance, meat quality, antioxidant capacity, and muscle fiber-related gene expression. A total of 750 one-day-old healthy 817 male crossbred broiler chickens were divided into five treatment groups, with six replicates per group: control group (CON), basal diet, basal diet with lipopolysaccharide challenge (LPS), basal diet with 150, 300, 450 mg/kg GA supplementation and LPS challenge (GA150, GA300, GA450). On days 14, 17, and 20, chickens in the LPS, GA150, GA300, and GA450 groups received intramuscular injections of LPS, while chickens in the CON group received saline. Compared with the LPS group, adding GA (GA150 and GA300) significantly increased the average daily gain (ADG) of broiler chickens from 1 to 50 days old (p < 0.05), and had the tendency to increase average daily feed intake (ADFI) (p = 0.078). Dietary addition of 450 mg/kg GA significantly reduced the pH value of breast muscle at 45min after slaughter (p < 0.05), and as the level of GA addition increased, the pH value first decreased and then increased. Compared with the LPS group, adding 450mg/kg GA significantly increased the brightness of breast muscles at 45min and 24h after slaughter (p < 0.001). Dietary supplementation of 450 mg/kg GA significantly decreased the total volatile basic nitrogen (TVB-N) content of post-mortem breast muscle (p < 0.05). Adding 450 mg/kg GA could significantly increase C22:6n-3 concentration in breast muscle (p < 0.05). Different GA levels had no significant effect on the intramuscular fat (IMF) content of broiler breast muscle (p > 0.05). Adding GA (GA150 and GA300) significantly increased (p < 0.05) the total superoxide dismutase (T-SOD) activity in the breast muscle after LPS stimulation, significantly increased (p < 0.05) the catalase (CAT) activity, and significantly reduced (p < 0.05) malonic dialdehyde (MDA) production, and had the tendency to increase glutathione peroxidase (GSH-Px) activity (p = 0.066). In addition, dietary supplementation of GA significantly increased the expression levels of myosin heavy chain (MyHC) I and MyHC IIa in breast muscles (p < 0.05), and significantly decreased the expression level of MyHC IIx (p < 0.05). In summary, dietary addition of GA can alleviate the stress response on the growth performance of broiler chickens and improve antioxidant capacity and meat quality. The appropriate amount of dietary GA at each stage was 300 mg/kg.