I have created a tar.gz file (using the GUI, not the command line). How can I update it with a command that new files are added and modified files are updated, too?
You'd normally use the -u flag for this. From tar's man page:
so this sequence will do what you need:
For a slightly more detailed look, see here: http://www.gnu.org/software/tar/manual/html_node/how-to-update.html