Recently, we reported that anticancer bioactive peptide (ACBP), purified from goat spleens immunized with human gastric cancer extracts, significantly inhibited gastric cancer cells in vitro and gastric tumors in vivo via repressing cell growth and promoting apoptosis, making it a promising potential biological anticancer drug. However, it is not known what genes are functionally required for the ACBP effects. Here, we first found that two tumor suppressor genes, cyclin-dependent kinase inhibitor 2B (CDKN2B) and growth arrest and DNA damage-inducible alpha (GADD45A), were upregulated significantly in the cells with ACBP treatment by microarray screening and the findings were validated by real-time RT-PCR. Next, GADD45A mRNA and protein expressions were downregulated in the gastric cancer cells by lentivirus-mediated RNAi; then, cell viability, cell cycle, and apoptosis were assayed by MTT and flow cytometry. Interestingly, our results indicated that cell viability was not dependent on GADD45A without ACBP treatment; however, cell sensitivity to ACBP was significantly decreased in ACBP-treated gastric cancer cells with GADD45A downregulation. Therefore, we demonstrate that GADD45A was functionally required for ACBP to inhibit gastric cancer cells, suggesting that GADD45A may become a biomarker for ACBP sensitivity. Our findings have significant implications on the molecular mechanism understanding, biomarker development, and anticancer drug development of ACBP.