G protein-coupled estrogen receptor (GPER) is known to play important roles in hormone-associated cancers. G-1, a novel synthetic GPER agonist, has been reported to possess anti-carcinogenic properties. However, the chemotherapeutic mechanism of GPER is yet unclear. Here, we evaluated GPER expression in various human gastric cancer tissues and cells. We observed that G-1 treatment attenuates GPER expression in gastric cancer and GPER expression increased G-1-induced antitumor effects in mouse xenograft model. We analyzed the effects of knockdown/overexpression of GPER on G-1-induced cell death in cancer cells. Increased GPER expression in human gastric cancer cells increased G-1-induced cell death because of increased cleaved caspase-3, -9, and cleaved poly ADP-ribose polymerase. Interestingly, during G-1-induced cell death, GPER mRNA and protein expression was attenuated and associated with ER stress-induced expression of PERK, ATF-4, GRP-78, and CHOP. Furthermore, PERK-dependent induction of ER stress activation increased G-1-induced cell death, whereas PERK silencing decreased cell death and increased drug sensitivity. Taken together, the data suggest that the induction of ER stresses through GPER expression may increase G-1-induced cell death in gastric cancer cells. These results may contribute to a new paradigm in gastric cancer therapy.