She usually sailed into battle accompanied by her sister ship Scharnhorst.
She carried a main armor belt of 350 mm (13.78 inch), comparable to modern battleships of the time, and vastly heavier than the World War I British battlecruisers HMS Renown and HMS Repulse and the French fast battleships Dunkerque and Strasbourg. The ships were armed with nine 280 mm (11 inch) main guns. While these had long range and quite good belt armor penetration power because of their high muzzle velocity, they were no match for the 380 mm (15 inch) guns of most of the battleships of her day. The choice of armament was a result of their hasty commissioning.
If a later proposal to upgrade the main armament to six 15-inch (380 mm) guns in three twin turrets, had been implemented, Gneisenau would have been a very formidable opponent, faster than any British capital ship and as well armored. When Gneisenau was designed, no 380mm guns were available for the German Kriegsmarine. It was decided to go ahead with 280mm guns, because as a commerce raider, she was not intended to fight a capital ship. Instead, superior speed would be used to avoid an engagement with a battleship. Due to priorities and constraints imposed by World War II, she retained her 11-inch guns throughout her career. Both Gneisenau and her sister were designed for an extended range to allow for commerce raiding.
She was considered a handsome ship, and looked as fast as she was. She and her sister ship, Scharnhorst, are generally spoken of as the most successful German design of the period. The main criticism of the design was their relatively low deck height above the water, the "freeboard", which made them "wet" when at heavy seas. This led to alterations in the sheer line and installation of the 'Atlantic Bow' in a winter 1938 refit. She conducted Battle training trials in the Atlantic in August-November 1938.
In April 1940, Gneisenau covered the invasion of Denmark and Norway, and along with Scharnhorst, battled HMS Renown without a decisive result. However, the ship suffered damage to her forward turret and her main gun director during the action. On 5 May, she set off a magnetic mine about 21 meters off her port quarter, and suffered shock damage, flooding, and loss of steering for 18 minutes. The damage was repaired by 21 May at Kiel. In the British withdrawal from Norway on 8 June, she and Scharnhorst surprised and sank the old British aircraft carrier HMS Glorious and her two escorts, the destroyers HMS Acasta and Ardent.
After repairs, she re-joined Scharnhorst in their most successful commerce raiding campaign - from January to March, 1941 (Operation Berlin) - with Gneisenau sinking 14 ships and Scharnhorst sinking eight, mostly from unescorted convoys. They avoided the British battleships operating as convoy escorts.
The two ships returned from the open Atlantic to the port of Brest, France, and then started preparations for their next operation. Gneisenau went into the dry dock for minor repairs. In early April, 1941, an unexploded bomb, dropped by RAF Bomber Command bombers during near constant air-raids on the ships, forced Gneisenau out of drydock, and she was anchored in the inner harbor. 22 Squadron of the RAF, a Coastal Command unit based at St. Eval was sent to attack Gneisenau. As a result, Gneisenau was torpedoed on 6 April 1941, reportedly by Flying Officer Kenneth Campbell. The damage was heavy and Gneiseau was put back into drydock - only to be further damaged by four aerial bombs on the night of 9 April-10 April. She underwent repairs at Brest through December, 1941.
In 1942, Gneisenau and Scharnhorst, accompanied by the heavy cruiser Prinz Eugen, and a covering screen of destroyers and torpedo boats, executed a daring daylight run to Germany, Operation Cerberus. All three of the major ships escaped damage in the furious air and sea battles that ensued in the English Channel. Gneisenau and Prinz Eugen severely damaged the destroyer HMS Worcester. Several salvos from Gneisenau destroyed the starboard side of the bridge. and the no. 1 and 2 boiler rooms. Prinz Eugen hit the destroyer a further four times, setting it on fire. Gneisenau later struck a naval mine off Terschelling, Netherlands, and she required repairs at Kiel.
One of the 38 cm guns intended for her rearmament exists today at the museum of Hanstholm in Denmark. This gun was planned as part of the German coastal battery "Tirpitz" at Oksby, Denmark, not far from the Blåvand lighthouse on the southwest coast of Jylland. The original 38 cm guns at Hanstholm (numbers 70,71,74 and 75) were destroyed during the 1950s. Her 28 cm guns from the turret called Anton were removed and sent to the Netherlands for use there; the turrets called Bruno and Cäsar and their guns were sent to Norway for coastal defence artillery there.
Her aft main turret, called Cäsar, was converted to a coastal battery named Austråt fort in Ørland near Trondheim, Norway, and it still exists today as a museum. The second turret called Bruno was stationed as a coastal battery at Fjell fortress near Bergen. Only the concrete base still stands. In Denmark, at the former "Stevnsfort" near Rødvig, two twin 15 cm turrets from her secondary armament still exist. In the Netherlands, parts of the guns of turret Anton are on display at the former "Stichting Fort", Hoek Van Holland.
(Promoted to KAdm on 1 April 1942.)