Gnome reads the icon name from the desktop file. You have to find the desktop file of this app, check its icon name there, and make sure there is a similarly named icon available in the icon pack.
To find the desktop file: open Looking Glass (Alt+F2 -> type lg
Enter-> click Windows on the top right) you should see your open windows there, it should show the name of the desktop file, even if you started from terminal. You can find the desktop file in ~/.local/share/applications
or in /usr/share/applications
. Open the file, and you should see a line starting with Icon
, this is what Gnome reads.
To check if a similarly named icon exists search for that name in /usr/share/icons/
. If you can't find a named icon, than the problem is in your icon pack, you should open an issue there. If you want to change the icon to something else, change the line in the desktop file.
More info in the glorious Arch Wiki: https://wiki.archlinux.org/title/Desktop_entries