A gap is generated in the weld joint due to dimensional error of welded parts and thermal deformation in aluminum alloy’s gas metal arc welding (GMAW) process of aluminum alloy. The optimum welding conditions corresponding to different gaps in the weld joint are required in the field. In this study, the welding conditions were optimized using Taguchi’s design of experiment method in response to gaps. Al5083-O with a thickness of 4.0 mm was used as the base material, and GMAW was performed on the T-fillet joints. An alternating current (AC) pulse was used for the welding process, and the welding experiment was performed for different gap sizes. Three levels of wire feed rate (WFR), electrode negative ratio (EN ratio), and teaching point (T.P) were selected as welding parameters, and 3 gap sizes (0, 0.5, and 1.0 mm) were selected as noise factors. Other welding conditions were fixed at a welding speed of 40 cm/min, a work angle of 40 degrees, a push of 10 degrees, the contact tip to work distance (CTWD) of 15 mm, and a shielding gas of 100 % Ar. The weld sizes such leg length, penetration depth, and throat thickness were measured using the cross-section. The maximum weld size that satisfies the minimum heat input was selected as the target value. Based on signal-to-noise ratio analysis, WFR, EN ratio, and T.P were selected as optimal levels of 11 m/min, 20 %, and 0 mm, respectively.