So you have a list of uncategorised urls currently? There are 2 ways you could use them in GSA.
1) Identify them (create an identified list) - from the site list tab in options there is a tools button, from that you click on import urls (identify...). This will start GSA off checking each url and determining each platform. At the end you should have an identified list.
or
2) Create a verified list straight away - create a new project, select the engines you're interested in using, fill out the project data as normal, make sure that options to use global site lists are unchecked, as are ALL the search engines. Then right click the project, import target urls, from file. Then run the project until it runs out of target urls. This will give you a verified list.
I always went with option 2 when I was a GSA user.