The most obvious choice for gz to pdf conversion would be exporting of GIMP images (.gz) as PDF documents (.pdf). The simplest way how to achieve this this, would be to download GIMP (it's free), open up your .gz images and use the PDF export function to produce the desired document.
However, on Linux platform, some users deal with the problem of converting downloaded ebooks in .gz format to readable PDF format. In such cases, you have to decompress the .gz archive first and then convert the ebook to PDF.Open GZ file Open PDF file
A free graphics and photo editor and authoring tool
GNU Image Manipulation Program
Mac version of the popular graphics editor