As of version 0.4.0.4, you can now enter a minimum distance and a maximum distance, and the program will show all gates whose distance to the chosen city is between the two values.
Note that you still can leave the first box empty and enter 500 in the second box, this would give you all cities within 500 miles distance from the chosen city. Or you can leave the second box empty and write 500 in the first box, this would give you all cities whose distance to your chosen city is more than 500 miles.
As of version 0.4.0.4, the locations of the previously misplaced cities of Edinburgh, Glasgow and Dublin have been corrected.
Many thanks go to styloboy for helping collecting the city data!