gslapt is just a GUI front end for downloading and installing packages. IMO before you start to use a program like it you would benefit from learning to compile and install from the command line. It's really easy and once you figure out how its done then you can move on to GUI like gslapt.
Also if I am not mistaken gslapt can be used with other mirrors but it was designed in mind to use with the slacky.eu repositories like the one in the link above, the actually URL is
http://repository.slacky.eu/slackware-12.2/. However slackbuilds.org has there own unofficial (I think) GUI frontend called "sbopkg" I would check it out also
http://www.sbopkg.org/.
check out this webpage (
http://www.slackbuilds.org/howto/)for more info on how to compile your own software using slack buildscripts. This is generally regarded as the best way to manage the software installed on your system since Pat himself uses buildscripts when building official packages