Whenever you select any file, GARbro attempt to read it and when it does, the file is cannot be modified, or archived by GARbro, hence you have to use move the file to an isolated folder and press control a to select everything without manually selecting them, causing the file to be "opened". Also should name archive file with patch.xp3, this modifies the game data without editing the data.xp3, main thing to consider doing that is that you do not include the initial folder, and only put the text files you modified.
You cannot manually select files in Garbro to archive, you have to use control all, then right click and archive so that not files are selected to be viewed,