Alka-seltzer vs gastric antacid

Alka-seltzer and gastric antacid both are nouns.

Word	Noun	Adjective	Verb	Adverb
Alka-seltzer	Yes	No	No	No
gastric antacid	Yes	No	No	No

As nouns, gastric antacid is a hypernym of Alka-seltzer; that is, gastric antacid is a word with a broader meaning than Alka-seltzer:

Alka-seltzer: a commercial antacid that contains aspirin; tablets dissolve in water to give an effervescent solution
gastric antacid: an agent that counteracts or neutralizes acidity (especially in the stomach)

Other hypernyms of Alka-seltzer include alkaliser, alkalizer, antacid, antiacid.

Alka-seltzer (noun)	gastric antacid (noun)
a commercial antacid that contains aspirin; tablets dissolve in water to give an effervescent solution	an agent that counteracts or neutralizes acidity (especially in the stomach)