Scribus is an extremely capable and highend oriented DTP app, comparable to Quark and InDesign. Stable and mature. Capable of publishing a newspaper or other 'real' publication with ICC color management, pre-press features, great PDF export, the works. Actually, on Ubuntu, you will probably want scribus-ng, which seems to stand for next generation. The regular scribus package is an earlier version that will probably not run on Ubuntu because of font recognition incompatibility. Check and enable the repos at the scribus web site and just install from apt or Synaptics.

Scribus is a print-oriented package and thus users will want to export in CMYK. The Gimp is know for not being able to do CMYK, only RGB. One of Gimp's main drawbacks as compared to Photoshop at this time. Ah but there is the gimp-cmyk plugin that allows you to put the four CMYK colors into layers and save as a TIFF, which can be imported into Scribus.