You’re conflating battles and wars. Battles are generally named after the locality in which they occur, or after the intention of the attacker (Battle of Midway, etc.). Wars name the whole conflict, and encompass the main theatre.

It’s also worth noting that these naming conventions almost always occur after the fact by commentators or historians.

The Battle of Gettysburg was a major engagement as part of the American Civil War. I don’t know enough about the conflict specifically, but it is unlikely that the belligerents referred to either at the time. Perhaps “civil war” was used descriptively. As for Gettsyburg, it would likely be a certain offensive, or a manoeuvre by a certain general, unit, or army. The belligerents don’t decide in advance to hold “the battle of Gettysburg” at a certain time and place. Engagements happen and names are assigned latterly to describe it specifically.

There are reported instances of belligerents naming an engagement upon their completion. Famously, and somewhat apocryphally, Henry V is alleged to have asked the name of the nearby town after his defeat of the French, and himself naming it the Battle if Agincourt, but this is likely a Shakespearean invention.

The Napoleonic Wars were not called such by Napoleon, nor was the 100 years war, thirty years war, or war of Spanish succession. All were named afterwards to describe a particular spell of hostilities.