Hi Daniel,
Geostatistical layers have very limited symbology options, as they are not really intended for display purposes. They are designed for quick analysis, visualization, and validation. But in a geostatistical workflow, you should nearly always export these layers to raster or feature for actual display purposes. The raster can symbolize at a fine scale (rather than a course grid that is used for geostatistical layers), and it can build its symbology based on its histogram (the geostatistical layer, on the other hand, cannot calculate a histogram for itself). We very strongly recommend to perform symbology with a raster rather than a geostatistical layer.
Also, the GA Layer to Contour tool can export your geostatistical layer to a polygon feature class that will look visually identical to the geostatistical layer. This tool has a classification value table where you can easily specify manual breaks. It can also be automated with Python.
