I am not familiar with Blender's C code and really don't know if I am on the right track here, but there is one function that seems to be called in all spaces that I can get gizmos working and not in spaces that gizmos don't work for me (tested in text editor and VSE): WM_gizmomap_tag_refresh()

This actually works for the preview area if you set

• bl_space_type = 'SEQUENCE_EDITOR'
• bl_region_type = 'PREVIEW'

but for some reason is missing in the "regular" bl_region_type = 'WINDOW'

Thank you so much! Using gizmos in the preview region is probably a more common use, but I didn't know about it. So I think you can also treat this as a matter of incomplete documentation.

I know this is not the right place to ask questions, but now that I have the attention of knowledgeable people on the rather obscure and new matter of custom gizmos: How do I make a 2D gizmo pan and scale with the area? It seems like in the node editor as well as the sequencer preview the gizmo is always drawn relative to the area frame and not to the content of the area.
I made a stackexchange question as well, but I have a feeling it will go unanswered: https://blender.stackexchange.com/questions/155528/python-how-to-pan-and-scale-gizmogroup-with-background

I havent really looked into this so will treat as known issue.
It may be very well bug, but I think this may easily turn out to accidental feature request