The adjective gastric comes directly from the Ancient Greek root gaster (gen. gastros) meaning stomach from the root gran to gnaw, eat, which in turn came from the Proto-Indo-European base *gras- meaning to devour. While the noun gaster does exist in English (meaning the enlarged part of the abdomen behind the pedicel in hymenopterous insects such as ants and termites), the word is so obscure in its substantantive form as to be virtually unknown.
The adjective is used in combination with words relating to the stomach: gastric juices, gastric bypass, gastric cecum. The Ancient Greek root gastro- is also common: gastro-intestinal, gastrodermal, gastroblast, etc. It even gave the oddly formed gastrocnemius (occasionally written as gastronemius) from the Ancient Greek gastroknemia which is the calf muscle in the lower leg-gaster added to kneme meaning leg, because the shape of the calf muscle resembled the shape of the stomach to Ancient Greek physicians.
Image of the stomach from Gray’s Anatomy, 1906 edition. Image from the 1906 edition now in the public domain.