Joppa is the location of the fictional alternate history epic American Civil War battle called The Battle of Gunpowder River in Grant Comes East: A Novel of the Civil War, the New York Times bestseller written by Newt Gingrich, William R. Forstchen, and Albert S. Hanser.
Gilmor's Raid was a military action that actually happened in the Civil War; the Battle of Gunpowder River written about in "Grant Comes East" was fictional.
Guess I won't look for any Civil War battlefields in the Gunpowder River area of Baltimore, Maryland, where my ancestors John Greer & Sarah Day lived since it was a fictional battle! [Here's a link to another descendant's file which is much more complete than my bare-bones entry for these ancestors.]
A related blog post here.