Post #572: 
You can create a new Panel Group with “My Grease Pencil Tab” category/tab,
add the panels to the group and use this code in Command tab:
from .scripts.command_panel import open_tab; open_tab("My Grease Pencil Tab")
Thank you, Roaoao. It has some problems, though. Here is a screenshot with my setup.
Screen Shot 1.jpg2300×1294 174 KB
Here is a screenshot when using one panel in UI
Screen Shot 2.jpg2300×1294 157 KB
Here is a screenshot with “My Grease Pencil” Button
Screen Shot 3.jpg2300×1294 157 KB
And here is what happen when I click “My Grease Pencil” Button… Please check the next post for the image.