Wasn't a "Napoleon" some sort of pastry?

Not during the Civil War ! There were several different types of field cannon developed prior to and during the war with many different nomenclatures. Four distinctive models were used in the field:

12-pounder bronze gun, Model of 1857

Commonly referred to as the "Napoleon", this bronze smoothbore cannon fired a twelve-pound ball and was considered a light gun though each weighed an average of 1,200 pounds. This powerful cannon could fire explosive shell and solid shot up to a mile and charges of canister up to 300 yards with accuracy. The Napoleon was a favorite amongst some Northern artillerists because of its firepower and reliability. Two Union batteries armed with Napoleons at Gettysburg were very effective in holding back Confederate infantry attacks and knocking down opposing southern batteries. Battery G, 4th US repeatedly slowed Confederate infantry attacks against the Eleventh Corps line on July 1 while Captain Hubert Dilger's Battery G, 1st Ohio Light Artillery almost annihilated two Confederate batteries with accurate and punishing counter-battery fire at long distance. Most Union Napoleons were manufactured in Massachusetts by the Ames Company and the Revere Copper Company. Confederate industry replicated the Napoleon design at several foundries in Tennessee, Louisiana, Mississippi, Virginia, Georgia and South Carolina. The Confederate design differed slightly from Union-made guns but fired the same twelve pound shot, shell and canister rounds used in Union manufactured guns.

2.9-inch (10-pounder) Parrott Rifle. This iron cannon was rifled and fired an elongated shell made specifically for the gun. Designed before the war by Captain Robert Parker Parrott, this gun was longer than a Napoleon, sleeker in design, and distinguishable by a thick band of iron wrapped around the breech. The Parrott design went through several improvements during the war and was changed in 1863 to a larger 3-inch bore and matching Parrott shell. The 3-inch Parrott was standardized the following year and most 2.9-inch guns were withdrawn from service. Parrott Rifles were manufactured by the West Point Arsenal in Cold Spring, New York and also made in 20 and 32-pounder sizes. The 10-pounder Parrotts used during the Gettysburg Campaign had an effective range of over 2,000 yards. The 5th New York Battery was composed of six 20-pounder Parrotts.
Confederate copies of the Parrott Rifle were produced by the Noble Brothers Foundry and the Macon Arsenal in Georgia. Parrott Rifles in 10 and 20-pounder sizes were sprinkled throughout some southern batteries.

3-inch Wrought Iron Gun

This sleek weapon was also called the 3-inch Ordnance Rifle and was designed by John Griffen, superintendent of the Safe Harbor Iron Works in Pennsylvania. The initial design was built by the Phoenix Iron Company of Phoenixville, Pennsylvania which manufactured most of the 3-inch Rifles used in the Union Armies. This iron gun was similar in length to the Parrott Rifle, fired an elongated shell, and was deadly accurate up to a mile. Much lighter than the Napoleon, the gun weighed an average of 800 pounds and could be easily transported and manhandled by its crew. Only a limited number of copies of the Ordnance Rifle were produced at Confederate arsenals.

Model 1841 12-pounder Howitzers. A pre-war bronze gun dating back to the 1840's, a number of howitzers were still in use by the Army of Northern Virginia during the Gettysburg Campaign. The barrels of these guns are several inches shorter than other artillery pieces giving them a stubby appearance. These powerful guns packed a whallop at close range but were not desirable for long range work. Larger 24-pounder field howitzers were also produced and though some appeared at Gettysburg, their use was mostly limited to forts and stationary defenses by this time of the war.

There were a number of other unusual guns in use by both sides during the campaign. Two of these are: