It might be a Windows 11 thing.
User installed fonts go in C:\Users\"yourname"\Appdata\Local\Microsoft\Windows\Fonts Is the Rockell Font There ? Gimp should pick up fonts there ? It does here in Win10.
Alternatives.
First the font files, instead of Micro$oft Store there are versions of the Rockwell font for download
https://freefontsfamily.org/rockwell-font-free/
https://font.download/font/rockwell-nova
Put the font files in your Gimp profile fonts folder: C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\fonts
User installed fonts go in C:\Users\"yourname"\Appdata\Local\Microsoft\Windows\Fonts Is the Rockell Font There ? Gimp should pick up fonts there ? It does here in Win10.
Alternatives.
First the font files, instead of Micro$oft Store there are versions of the Rockwell font for download
https://freefontsfamily.org/rockwell-font-free/
https://font.download/font/rockwell-nova
Put the font files in your Gimp profile fonts folder: C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\fonts