In gimp in order to add a copyright symbol © inside a text, you can use the following trick both for Windows as well as Linux.
Press Ctr-Shift-u then type a9 and press enter. It’s a weird way but it works!
Update: Those who are seeing boxed u’s in your image, do not worry its a gimp GUI bug, it isn’t really part of your image. You can safely save your changes and ignore the boxed u’s. You will notice they remain on the gimp screen even if you close the image. Just restarting Gimp will permanently remove it.