This feature was available in the previous version of Gavagai Explorer and will return soon. In the mean time, as a workaround you can export the results to Excel and then sort in Excel.
First of all you must export your project as an Excel file. This is done by clicking save as > Excel file in the top left panel of the project's Explore screen.
When this file is prepared and you have opened it, you will see the "summary" tab by default, showing you a similar overview of to what is visible when you are exploring your project in the graphical interface.
Using the normal Excel Sort function, you can then sort the Topics by the values given in these columns and thus see the texts which receive the highest scores for negativity or positivity, the most negative or skeptical texts according to the system's sentiment analysis, depending on which column you sort by.
Here is a link to the how to understand the Excel file itself.