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.
Corn spirit is American origin. During the era of European exploration corn was one of the first crops that they took back to their home continent.
In the modern day, many gin distillers are playing with corn as a base. Firstly, because it is easily and widely available. Grown to surplus levels, especially stateside it’s an easy source of fermentable sugar. Secondly, if a distillery is also distilling a bourbon— which legally requires at least 51% of the mash bill to be corn— they probably are bringing a great deal of it in.
But thirdly and most importantly— many people choose a corn base because of the taste and texture. Corn spirit is often described as being slightly sweeter in character, especially on the finish than other grains. Corn spirit can also have residual character suggestive of— well corn. Corn chips, masa dough, or creamed corn aren’t unheard of as tasting notes for corn spirit.