Gastric duplication cysts (GDCs) in adults are exceedingly rare, with only a few documented cases in medical literature. The spectrum of clinical presentations varies widely, ranging from asymptomatic to severe symptoms such as hematemesis, vomiting or abdominal pain. Among the less common complications associated with GDCs, segmental portal hypertension is a notable rarity. We present a compelling case report of a patient exhibiting signs of segmental portal hypertension, where ultrasound and echo-endoscopy revealed a sizable gastric duplication cyst as the underlying etiology. Recognizing the scarcity of literature on GDCs in adult patients, we conducted a thorough review to underscore the diagnostic significance of ultrasonography and endoscopic ultrasound (EUS) in accurately identifying these congenital anomalies. This case report and comprehensive literature review emphasize the pivotal role of EUS and abdominal ultrasound in achieving an accurate diagnosis of GDCs. By shedding light on the diagnostic and therapeutic intricacies, we aim to raise awareness among clinicians regarding this rare pathology and the importance of multimodal imaging approaches for optimal patient management.