Dense granule protein 16 (GRA16), an effector protein from Toxoplasma gondii, has been reported to enhance antitumor responses in several malignancies. However, whether GRA16 can enhance the inhibitory effects of standard anticancer drugs across diverse cancer types remains unclear. Gemcitabine, oxaliplatin, and irinotecan continue to play a key role in clinical oncology; however, their efficacy is limited by persistent tumor survival-promoting pathways. In this study, we stably expressed GRA16 in HepG2, PANC-1, and HCT116 cells and evaluated their responses to gemcitabine, oxaliplatin, and irinotecan under 50% inhibitory concentration-normalized dosing conditions. In vector control cells, baseline drug sensitivity was quantified using the CCK-8 assay across 0–64 μM after 48 h of exposure, and 50% inhibitory concentration was determined for each drug and cell line. Time-dependent proliferation assays (0–72 h) demonstrated that GRA16 reduced the basal growth rate and enhanced the growth inhibitory effects of all 3 drugs in all 3 cell models. Representative microscopy images at 48 h were consistent with reduced cell density in GRA16-expressing cultures under each treatment condition. These in vitro data indicate that GRA16 enhances the inhibitory effects of standard chemotherapeutic agents in hepatic, pancreatic, and colorectal cancer cell lines. This study provides a cross-cancer, multi-drug evaluation of GRA16-associated enhancement of drug response and includes, to our knowledge, the first assessment in a pancreatic cancer model. The underlying mechanism may involve pathway modulation, as suggested by previous studies and supported by the pancreatic cancer model although it was not comprehensively examined across all cell lines.