The Gap value supplied will usually be rounded to the nearest "nice" value, suitable (e.g.) for generating axis labels, before use. To avoid this "nicing" you should set an explicit format for the axis using the Format(axis) or Digits/Digits(axis) attribute. The default behaviour is for the Plot to generate its own Gap value when required, based on the range of axis values to be represented.
AST A Library for Handling World Coordinate Systems in Astronomy