I can explain what is happening and suggest some solutions, but not provide a solution for your precise question. But one of my solutions is very close.
Finder and Spotlight:
In "as Gallery" mode Finder shows a subset of the metadata which has been indexed by the Spotlight importer for jpeg files. As you have discovered this includes at least some EXIF data, but not any IPTC. And you are not able to extend what Finder displays to include all EXIF and IPTC metadata.
You can show what Spotlight has indexed with the terminal command:
mdls <path to file>
I have taken one of my photos and (using Graphic Converter) added the IPTC Title field as "Flowering Plum". This is indeed indexed by Spotlight as shown with:
mdls "/Volumes/Photos/PhotoSync/Gilby’s iPhone/Recents/IMG_3495.HEIC"
_kMDItemDisplayNameWithExtensions = "IMG_3495.HEIC"
kMDItemAcquisitionMake = "Apple"
kMDItemAcquisitionModel = "iPhone 13 Pro Max"
kMDItemEXIFVersion = "2.3.2"
kMDItemExposureMode = 0
kMDItemExposureProgram = 2
kMDItemExposureTimeSeconds = 0.001366120218579235
kMDItemFlashOnOff = 0
kMDItemFNumber = 1.8
kMDItemFocalLength = 1.57
kMDItemTitle = "Flowering Plum"
I have deleted much of the output, but shown some EXIF metadata and, more importantly, the IPTC Title field has been used to populate
The bad news is that this doesn't change what Finder displays in Gallery mode.
The good news is:
- Finder's Get Info (Command-I) does display the KMDItemTitle metadata as "Title: Flowering Plum".
- You can use Command-Option-I to display a Get Info window which remains in place and is populated by the Spotlight metadata for the currently selected file.
- A Finder search for "plum" will find the photo.
Here is a snippet from the Get Info:
Preview: I am pretty sure that Preview does look directly at the metadata inside the file and displays that directly.
Apple Photos: When I import my test photo into Photos (macOS 12.5), it is displayed with the title. The Title is also shown in its "Info" panel.
Graphic Converter, Lightroom or other Digital Asset Manager: These do, of course, display all metadata.
In summary you can:
- Use Finder's search and it's "Get Info" panel to show use/show the Title metadata - but not in Gallery view's right hand info panel. I don't see a way round the restriction.
- The closest to what you ask is to use the permanent Get Info window activated by Command-Option-I.
- Use Apple Photos as a Digital Asset Manager.
- Use a third party DAM.
Everything I have said for the IPTC Title, also applies for the IPTC Caption which appears in the Spotlight kMDItemDescription metadata and is displayed in Finder's Get Info as "Description".