These files are zipped and must be unzipped with a zip compatable decompress software such as winzip. When unzipped these files are GWK files and interface to a free gerber viewer known as GCPreview® from Graphicode. You may obtain the reader by clicking on the link "Go Get Graphicode CGPrevue® Free Gerber Viewer" .   Go to the download area and pick GCPrevue®.  Install the software and open the file.