3 hours ago
(This post was last modified: 2 hours ago by gophersnake.)
I downloaded Rockwell from the freefontsfamily.org site, unzipped the result in the downloads folder, and copied the two .TTF files to C:\Users\[My username]\AppData\Roaming\GIMP\2.10\fonts. When I launched GIMP again, I found two new fonts in my fonts list: Rockwell and Rockwell Bold. Original problem solved, it looks like! 
I'm still speculating (more or less idly) about the likely implications. Yesterday, after I'd downloaded the Rockwell Nova fonts from Microsoft:
-- Windows 11 showed Rockwell Nova on the Settings > Personalization > Fonts page but not in the C:\Windows\Fonts folder.
-- Open Office Writer showed the whole Rockwell Nova family in its font selector pulldown.
-- GIMP wasn't finding any Rockwell fonts.
Today, with the newly-downloaded Rockwell fonts in GIMP's fonts folder, it did find them. This suggests to me that there must be at least two ways for an app to access collections of fonts. One way is by looking in the fonts folders, as GIMP apparently does. The other way must be whatever Open Office does -- somehow querying Windows for fonts, I'd imagine. That second way can't be very new; it must have been available under Windows 10 as well. The Open Office that I'm using hasn't been updated since2022 2018.
Windows 11 seems to be phasing out the old way to access fonts but GIMP doesn't seem to be making use of the new way yet.
I'm still speculating (more or less idly) about the likely implications. Yesterday, after I'd downloaded the Rockwell Nova fonts from Microsoft:
-- Windows 11 showed Rockwell Nova on the Settings > Personalization > Fonts page but not in the C:\Windows\Fonts folder.
-- Open Office Writer showed the whole Rockwell Nova family in its font selector pulldown.
-- GIMP wasn't finding any Rockwell fonts.
Today, with the newly-downloaded Rockwell fonts in GIMP's fonts folder, it did find them. This suggests to me that there must be at least two ways for an app to access collections of fonts. One way is by looking in the fonts folders, as GIMP apparently does. The other way must be whatever Open Office does -- somehow querying Windows for fonts, I'd imagine. That second way can't be very new; it must have been available under Windows 10 as well. The Open Office that I'm using hasn't been updated since
Windows 11 seems to be phasing out the old way to access fonts but GIMP doesn't seem to be making use of the new way yet.