blender 2.8 a421cfa8d31
Grease Pencil gizmo of a separated stroke not selectable by cursor, selection by Select Box works.
1: Add a Grease Pencil stroke, or a blank Grease Pencil and then draw a stroke.
2: Enter Edit mode, select all and duplicate (shift+D), then select only one stroke and P to Separate-Selected Points
3: Go to Object Mode. You can't select the gizmo of the newly created object, you can select it by using Select Box but if you go in edit mode it makes you edit the original stroke.