The effect of supplementation of γ-aminobutyric acid (GABA), a component of the neuroendocrine cascade involved in stress alleviation, was studied in broiler chickens in a heat stress model. In a 42-day feeding trial, 300 male and 300 female broiler chickens were equally distributed in an environmentally controlled (EC) and an open house (OH) according to a 2x2x3 factorial design. The diet was either devoid of GABA (0 mg/kg) or was supplemented with 60 or 120 mg/kg GABA. Zootechnical performances (1-42 d), expression of mRNA for genes coding for inflammatory cytokines (IL-1β, IL-8, TNF-α) in spleen (24 d), oxidative regulators (Nrf-2, SOD) in liver (24 d), and arterial blood gas concentrations (24 d) were assessed. Whole genome sequencing of the caecal microbiome was performed for the male birds (42 d). Supplementation of GABA improved (P < 0.01) body weight and feed conversion ratio irrespective of sex and housing conditions, expression of IL-1β and IL-8, and Nrf-2 gene. GABA decreased (P < 0.05) malondialdehyde and increased (P < 0.05) glutathione peroxidase activities in serum. GABA decreased circulatory corticosterone concertation (P < 0.05). While both housing conditions benefitted from GABA, its effects were more pronounced in OH (house effect P < 0.05). GABA supplementation modulated caecal microbiota composition by increasing the relative abundance of beneficial bacteria such as Alistipes and Bacteroides, particularly in the OH environment. Overall, this study concludes that 60 mg/kg GABA supplementation improves growth performance, stress resilience, and microbial composition in broiler chickens, with variable effects depending on housing conditions.