Following is an excellent two part series on gLabels by Nathan Willis: Getting started with gLabels and Advanced usage with gLabels.
Labels and Cards with gLabels (Part One)
In the world of label creation software for Linux, gLabels is the long-standing market leader. It offers a convenient graphical interface in which you can design labels with the same tools you are used to finding in image editing software, but it also supports business-friendly advanced features like “mail merge” and barcode generation. In addition to that, its focus on label creation offers some advantages in printing over general office or graphics alternatives, like simple control over printing partial sheets.
You can download source code for gLabels from its SourceForge project page, but most Linux distributions include it in their package management systems — the gLabels site maintains a list of such distros on the download page. gLabels uses GTK and is designed to work with the GNOME desktop environment, but it runs just as well under KDE.