A phrase like decades of study sounds reassuring. It deserves a closer look.
What the claim can mean
Berberine and gymnema sylvestre have both been studied in various ways over many years, and that is a reasonable thing to say about the ingredients in general.
What it cannot mean
It does not mean GlucoPril itself has been tested. Study of an ingredient is not study of a specific product at these undisclosed amounts. The sources the page links are search pages and a fact sheet, so read them yourself rather than relying on the headline.
Educational note: This guide is for general information only. GlucoPril is a dietary supplement formulated to support healthy glucose metabolism and is not intended to diagnose, treat, cure, or prevent any disease.