Steps to reproduce:

1. Open attached project "851010.zip"
2. Open scene "test"
3. Notoice that GameObject has two Gizmos draw. MonoBehaviour UsingDrawGizmoAttribute draws Cube gizmo using DrawGizmo attribute. WithOnDrawGizmosMethods MonoBehaviour draws sphere gizmo in OnDrawGizmos
4. Expand Gizmos dropdown in game view and notice that there no custom gizmo called UsingDrawGizmosAttribute

This issue occurs because Editor script used to draw gizmo isn't CustomEditor, doesn't extend Editor class and also doesn't have OnSceneGUI method. Adding CustomEditor tag, making class to extend Editor and also implementing OnSceneGUI makes gizmo appear in Gizmos dropdown list.

Reproduced with: 5.2.4f1, 5.3.7f1, 5.4.2p4, 5.5.0f1, 5.6.0a3