Extract all the county files from the ZIP file into a directory of your choice on your computer.
There are 29 county files, plus this README file.

In GSAK, you can create a filter for any county by doing the following:

1. Bring up GSAK.
2. From the dropdown menu select: Search --> Filter...
3. Select the Arc/Poly tab.
4. Click on the 'Load from file' button.
5. Find the county file for which you wish to create a filter, and select 'Open'.
6. In the box labeled 'Filter Type' select 'Polygon'.
7. Press the 'Save' button at the bottom and name your filter.
8. Click on the 'Go' button to run the filter.