A simple Blender add-on to modify the length (exposure) of greasepencil frames.
Go to Edit > Preferences > Add-ons > Install... then navigate to the greaspencil_nudge_frames folder. Select greasepencil_nudge_frames.py and confirm, then enable it with the checkbox.
With the mouse over the Grease Pencil Dopesheet, press:
Dto shorten by 1 frameFto extend by 1 frame
The shortcuts change the length of the displayed frame(s) for every unlocked layers in all selected GP objects. Make sure you isolate the layer you want to affect or lock the ones you want to keep in place.
I tried to make sure that frames aren't overwritten by accident, so when the current keyframe in any unlocked layer (even if hidden) is only one frame away from the next, the D shortcut is disabled. Even so, this is my first add-on so use at your own risks and all that.