It has a very simple GUI and comes with a toolbar of its own which contains Text utility (change font size and colors, etc), change resolutions, add different colors, load pictures, etc. It also supports CSV type file importing, so you can add them directly to the cover project with ease.
gLabels has a built in wizard type interface and you can choose predefined sizes for printable covers/documents or can manually define your own sizes.
This is officially supported by Ubuntu repositories and by using the below command you can install gLabels in Ubuntu 10.04/10.10 and 11.04 Natty Narwhal.
sudo apt-get install glabels
Although they recently updated to a never version 3.0 which adds the support for the never Gnome3/GTK3 desktop + numerous improvements, it's not added to the repositories, yet. You can compile it and install if you can't wait :) but that would require GTK3 and few other dependencies, so it's better to wait till it's supported.