Gins which feature a Neutral Grain Spirit— often shortened to NGS— don’t specify the grains in their base. Neutral grain spirits is also known as “pure grain alcohol”. It’s concentrated to the point of having no residual flavor or character. Often distilled to 95% Alcohol by volume, it’s the purest ethanol that is physically possible through traditional distillation.
Many high quality gins start from a base of NGS. Starting from the most neutral base possible means that distillers have a truly blank canvas from which to begin with for their botanicals.
NGS, neither the presence nor lack thereof indicates anything about the quality of the gin in the bottle.