I tried to put information about the publication of images from additional gallery (gallery field) on the default Activity TAB.
It was not easy, after three days of searching CB forum and reworking various autoactions, manipulating of assets only achieved the publication of the post, but without the image, which I added as a url link of the type.The result, of course, without the Modal window opening with comments as in the gallery...

In the end, I found the solution a bit by accident:
1. Create a Gallery type field in any TAB and note the field id (FIELD_ID)
2. Leave this new Gallery's Assets empty - media will have default Asset: profile.USER_ID.field.FIELD_ID
3. Save as a copy of CB Auto Actions: System Actions -> CB Activity, CB Gallery - Create
4. In this new autoaction:
a. We leave the settings including Triggers (galery_onAfterCreateItem, galery_onAfterUpdateItem)
b. Change the Condition (asset from our new gallery field)
VALUE [var1_asset]
Equal To
profile.[user_id].field.FIELD_ID

c. And now the most important thing (which I discovered being a bit desperate) in Action Asset we combine what has a standard gallery with what has a field gallery, that is:

Asset: profile.[user_id].field.FIELD_ID.gallery.[var1_type].[var1_id]

And that's it. It works perfectly.
The grouped pictures from both galleries appear on the Activity stream separately with the Modal window working in both cases after clicking. And when you remove an image from an additional gallery, the standard Autoaction system also deletes the entries from the new Gallery field.

I hope it will be useful to someone.

Thanks!
Confirm: In last CB Gallery, Version: 2.5.0+build.2021.06.11.13.15. it works out of box. Thx again.