Purpose <p>This study aimed to evaluate whether different tumour response assessment methods yield inconsistent results following neoadjuvant chemotherapy (NACT) in patients with gastric cancer and to identify the methods most strongly associated with pathological response and prognosis.</p> Methods <p>This study included 217 patients with gastric cancer who underwent NACT. Tumour volume, area, and thickness were measured on baseline and post-treatment computed tomography (CT) scans. Reduction rates (%Δ measurement) for each parameter were calculated, and the concordance between different %Δ measurements was assessed. Tumour regression grade (TRG) and overall survival (OS) data were recorded, and the predictive performance of the assessment methods was evaluated.</p> Results <p>The concordance between different %Δ measurements ranged from fair to moderate (κ = 0.26–0.50), with 24.88–36.87% of patients showing discordant treatment response assessments across methods. The area under the curve (AUC) values for distinguishing pathological responders from non-responders were 0.70 for %Δ volume, 0.75 for %Δ area, and 0.67 for %Δ thickness. For OS prediction, the c-index values were 0.60 for %Δ volume, 0.59 for %Δ area, and 0.54 for %Δ thickness. Additionally, discrepancies were observed between %Δ measurements and TRG. Both %Δ volume and %Δ area were effective in stratifying OS in patients with TRG 2–3.</p> Conclusion <p>Tumour response assessment methods showed discordance across different measurements, with volumetric and planimetric measurements offering superior accuracy than linear measurements in predicting both pathological response and OS. Additionally, %Δ volume and %Δ area were effective in stratifying OS in patients with TRG 2–3.</p>