You need a svn client program to check out the source code for projects hosted on http://code.google.com. Most Linux systems should already have svn client installed and the following command should work:

svn checkout http://gmmreg.googlecode.com/svn/trunk/ gmmreg-read-only

Windows users are recommended to use TortoiseSVN. For how to use TortoiseSVN, there is a nice introduction by limodou, the author of UliPad.

Note that the "URL of repository" for this project should be

http://gmmreg.googlecode.com/svn/trunk/