Richmond would qualify as a city, I would think. But Atlanta, for instance, was the 99th largest "city" in the top 15 in the CSA.
1860 Census:
Rank City State Population Notes
1 New York New York 813,669
2 Philadelphia Pennsylvania 565,529 The large jump in population between the seventh and eighth censuses is due to the 1854 Act of Consolidation, which merged the County and City of Philadelphia into a single government entity and abolished all other local governments.
3 Brooklyn New York 266,661 Brooklyn would cease to be a city in its own right in 1898. It is now a borough of New York City.
4 Baltimore Maryland 212,418
5 Boston Massachusetts 177,840
6 New Orleans Louisiana 168,675
7 Cincinnati Ohio 161,044
8 St. Louis Missouri 160,773
9 Chicago Illinois 112,172 First appearance on top 10. In the previous census, it was the 24th largest American city with a population of 29,963. Chicago would be the world's fastest growing city during the entire 19th century.
10 Buffalo New York 81,129 First appearance on top 10. Would disappear from list by next census and not re-appear until 1900.
Here's a more complete list:
http://www.amartcivilwar.org/Largest_Cities_1860.htm
Now, your criteria still needs to be clarified: Gettysburg really wasn't "involved" in the war save for 3+ days where the armies happened to meet.
As far as involvement being "Importance", there again you'd be looking at those larger cities because they contributed more men and materiel.
That is also not strategic importance, because then you'd need to include all major southern ports, including those on the Mississippi River and elsewhere. Gettysburg could arguably be on that list due to the large road junction, but it was never important in and of itself (as a strategic or tactical target).