GS markers are mainly useful in the object snap implementation of an entity. When the entity is selected for object snap, the GS marker for the selected portion of the entity is passed back to indicate which points should be returned.
GS markers are also used in conjunction with the functions acedSSGet() and acedSSNameX() to permit your application to edit or operate on arbitrary sections of your custom entity objects. For a detailed description of how to use GS markers (not how to set them), including the use of the acedSSGet(), acedSSNameX(), and AcDbEntity::getSubentPathsAtGsMarker() functions, see GS Markers and Subentities.
The examples in Entities, set a GS marker for every edge of the entity. Your custom entity can use markers to identify a set of arbitrary sections of the entity—that is, any sequentially executed group of primitives can be identified by a single marker. The section of the entity generated by the group of primitive function calls is identified by preceding the primitives with a call to the AcGiSubEntityTraits function setSelectionMarker(), specifying a non-zero marker number unique to the entity object. Your implementation of getSubentPathsAtGsMarker() will associate the appropriate primitives with a given marker, based on how you set your markers. A selection marker value of zero has special meaning to the AutoCAD display system and should not be used.