The base spirit of a gin refers to the distillate to which botanicals were added. It’s helpful to think about the base spirit as a vodka-like spirit that the distiller used as a starting point— a blank canvas upon which the gin was designed.
But choice of base alone doesn’t mean that you’ll be able to taste it. Depending on choices the distiller makes, such as how many times the base is distilled, or to what strength it is distilled to— there might be no character at all coming from the base spirit.
As gin is legally defined by the inclusion of juniper as a botanical, distillers are free to choose any base spirit for their gin that they would like. Distillers aren’t legally required to disclose on the bottle the precise nature of their base spirit; therefore, sometimes we’re left wondering.