Stomach acid is necessary to properly digest and absorb nutrients from food. Low stomach acid is a much more common problem than high. Reflux/heartburn (GERD) most likely indicates low acid, because the valve between the stomach and the esophagus isn't signaled to close properly. Reflux medication inhibits the production of stomach acid - which prevents acid from entering the esophagus, but causes malabsorption and increased susceptibility to bacterial overgrowth and parasites.
Zinc is necessary for the enzyme that makes stomach acid (carbonic anhydrase), and isn't freed from food well without adequate stomach acid.
So low zinc is highly corelated with low stomach acid.
Gastrin is a hormone produced in the stomach that promotes release of stomach acid and closing of the lower esophageal valve. So low gastrin might cause GERD symptoms. Gastrin is less active when you don't have adequate sulfate (healthgeneration.com).
Reasons to Suspect
If eating beets turns your urine red, reflux, bacterial overgrowths, malabsorption, low zinc, B12 malabsorption
Zinc and water are required for the enzyme that makes stomach acid.
Histamine stimulates the release of stomach acid. Histamine is found in fermented foods as well as those listed here.
Protein and calcium (and many other things) stimulate gastrin secretion.
Thiamine may stimulate stomach acid as well.
Antacids will, by definition, lower stomach acid. They should be avoided unless there is strong evidence indicating that they're necessary. I'm thinking along the lines of stomach ulcers, but haven't done enough reading to really say anything definitive.