Oral intake of gamma-aminobutyric acid (GABA) reduces stress and promotes relaxation, but its effect on esports performance is unclear. In a randomized, double-blind, placebo-controlled crossover trial, 21 MOBA-genre esports players were enrolled; 18 were analyzed after prespecified exclusions. Thirty minutes after ingesting 200 mg GABA or placebo, players completed the Mobalytics Proving Ground (MPG), a standardized MOBA micro-performance battery. Frontal EEG at Fp1 yielded an SMR-weighted, EEG-derived engagement index, a putative EEG marker of task engagement (flow). GABA increased total MPG score and the engagement index versus placebo (p < 0.05). Higher ΔSMR was associated with higher MPG scores (r = 0.418, p = 0.011), and the ΔSMR × treatment interaction was not significant (F(1,16) = 0.367, p = 0.553). These findings indicate that oral GABA intake can enhance immediately micro-management performance under cognitive load accompanied by increasing EEG-indexed engagement in esports.