I’m not sure how you managed to do this, but the Grease Pencil object(Stroke) is active, but isn’t selected. Click on the Stroke object in the outliner(It should highlight with blue) and then you should see the keyframe. If you don’t then you may have filtering in dopesheet. With mouse cursor over timeline editor press Shift+F12 or change the editor type from timeline to dopesheet in first icon on the header. Click on the funnel icon and make sure that grease pencil is turned on(Blue).
