In previous releases, Grease Pencil was a property of scene.
It was not at level of object data as it is in 2.8.
So, the fact that Grease pencil keyframes are displayed on a special mode is inherited from previous situation of Grease Pencil.
In 2.79, as scene data, there was no dependency visible in dopesheet.
Now, there is no reason to keep Grease Pencil keyframes into a separated mode and not expose it like other Object Data (Shapekeys, Mesh Data, Material or Modifiers) on same graph than Transforms.
That is not the unique problem. There are also not visible into Action Editor mode and NLA editor.
A time offset modifier was added to attenuate this issue.
One of developer of Grease Pencil is also developer of animation system. He is aware of that.
If it is not solved for 2.80, because of a lack of time to properly solve those issue before delivery a stable version of 2.8. There is no doubt that, at moment, consistency of exposure of animation data will be restored in future releases.