This is all that is needed to get Apple HEIC image thumbnails in the Gnome file manager. I am currently running Ubuntu 23.04 but this should be similar in most distros using their package managers. It’s so easy a caveman could do it…
sudo apt update
sudo apt install libheif1 heif-thumbnailer heif-gdk-pixbuf
That’s it. You may need to log off and back on. I rebooted.
If you want an easy Bash script to convert HEIC files in batch, look here.