List all subcategories of particular categories in CPT

You would need a custom function for that.

The available options for GDV1 are shown here:

https://wpgeodirectory.com/docs/core-shortcodes/#popcats

It is not possible in V2 either without a customization.

GD Categoris for GDV2 has a lot more options than in GDV1, but it is still not possible to limit the selection to a particular parent category.

Can you say more about what you are trying to do?
Because if you are looking for a query then you can open up the plugin and look in a number of places to see how category queries are done.

Most members want to display either all the categories (on location page = browse all) or just the assigned categories (for example on a detail page = browse similar).

I can picture what you are looking for in my head, but, where would you use it? on a detail page? Based on the category of the detail page? Or just trying to show a certain specific category?

Well, that is what GD Categories does for V1 and V2 but you cannot single out a parent category.

Try it out and let us know what you think:

https://wpgeodirectory.com/docs-v2/geodirectory/design-elements/#gd_categories