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.
Gin is rather unique among spirit categories. While many spirits are defined by the type of distillate— for gin this is not true. Gin can have any base spirit, and as long juniper is predominant. It’s gin.
One Academic writer wrote a piece wondering why the British peoples never developed Calvados, a brandy distilled from fermented apples, given the apple’s widespread presence on the British countryside. The nation’s obsession with grain despite the vast amount of other fermentable sources suggests that it could have easily gone the other way. Meaning that if they leveraged their local resources, the English adaptation of Dutch Coverage might have been distilled from ciders, rather than ales.
Apple base can vary based on the type of apples used and the place where its used. I know some spirits writers describe a certain distinctive earthy note of Upstate New York apple base spirits as affectionately having a “New York funk.” Given their wide variation and distinctive character, many distillers choose to build around the apple base spirit and use its unique character as an asset.