gastric acid vs digestive juice

gastric acid and digestive juice both are nouns.

Word	Noun	Adjective	Verb	Adverb
gastric acid	Yes	No	No	No
digestive juice	Yes	No	No	No

As nouns, digestive juice is a hypernym of gastric acid; that is, digestive juice is a word with a broader meaning than gastric acid:

gastric acid: digestive secretions of the stomach glands consisting chiefly of hydrochloric acid and mucin and the enzymes pepsin and rennin and lipase
digestive juice: secretions that aid digestion

Other hypernyms of gastric acid include digestive fluid.

gastric acid (noun)	digestive juice (noun)
digestive secretions of the stomach glands consisting chiefly of hydrochloric acid and mucin and the enzymes pepsin and rennin and lipase	secretions that aid digestion