OK so the author should be the name of the organization that wrote the program if applicable and the website should be where the source can be downloaded. Only question is why is License GPL when the copying file in the source says it's under a GNU license.
To answer your question (Also Thanks Ules for the tip)
GPL is known actually as GNU/GPL we shorten it to just GPL. sometimes you'll find LGPL and a few may also be under BSD
Now if you gotten this far saved the file. You need to copy the gpsk31-0.5-tar.gz file into the src folder.
once that's done your ready to build it.
In a terminal su to root > then cd to the /gpks31/0.5/src folder in this folder type the following # sh ./gpsk31.SlackBuild
the script you created should run. if all goes well and it finishes without any cofigure errors or make errors you'll find a few new files in the /gpks31/0.5 folder you'll have a Slackrequired file a slackdiscription file and a .tlz package the .tlz is the official VL package. you can install this package by right clickon it and selecting the install pkg prompt.
If the script runs and you get an error that you need such and such a file.. go to gslapt see if it's available and install it and run the script again. If that dependency is not available in gslapt you would have to go find it's source code (Google is your friend here) and build that package and install it first. then re-run the build script for what ever program your building.
If this all works we will move on.
let me know.