Objective: Radiomics, an emerging tool for medical image analysis, is potential towards precisely characterizing gastric cancer (GC). Whether using one-slice 2D annotation or whole-volume 3D annotation remains a long-time debate, especially for heterogeneous GC. We comprehensively compared 2D and 3D radiomic features' representation and discrimination capacity regarding GC, via three tasks (T ^LNM , lymph node metastasis' prediction; T ^LVI , lymphovascular invasion's prediction; T ^pT , pT4 or other pT stages' classification). Methods: Four-center 539 GC patients were retrospectively enrolled and divided into the training and validation cohorts. From 2D or 3D regions of interest (ROIs) annotated by radiologists, radiomic features were extracted respectively. Feature selection and model construction procedures were customed for each combination of two modalities (2D or 3D) and three tasks. Subsequently, six machine learning models (Model ^LNM _2D , Model ^LNM _3D ; Model ^LVI _2D , Model ^LVI _3D s Model ^pT _2D ,s Model ^pT _3D ) were derived and evaluated to reflect modalities' performances in characterizing GC. Furthermore, we performed an auxiliary experiment to assess modalities' performances when resampling spacing different. Results: Regarding three tasks, the yielded areas under the curve (AUCs) were: Model ^LNM2D 's 0.712 (95% confidence interval, 0.613-0.811), Model ^LNM _3D 's 0.680 (0.584-0.775); Model ^LVI _2D 's 0.677 (0.595-0.761), Model ^LVI _3D 's 0.615 (0.528-0.703); Model ^pT _2D 's 0.840 (0.779-0.901), Model ^pT _3D 's 0.813 (0.747-0.879). Moreover, the auxiliary experiment indicated that Models _2D are statistically advantageous than Models _3D with different resampling spacings. Conclusion: Models constructed with 2D radiomic features revealed comparable performances with those constructed with 3D features in characterizing GC. Significance: Our work indicated that time-saving 2D annotation would be the better choice in GC, and provided a related reference to further radiomics-based researches.