Having read and watched a lot about the first world war this year, I haven't heard or read anything about the allies ever considering landing forces behind enemy lines somewhere in Europe.
I'm aware this doesn't mean they "never considered it", but I still find it weird that it's not mentioned at all.
What was the reasoning for not launching Operation Gallipolli somewhere along the Baltic coast, or better yet, somewhere along the North Sea coast?
Netherlands was neutral, so landing there was probably impossible without dragging them into the war, but there are tons of room around Bremen and one would think that flanking the Belgian part of the German forces wouldn't be to far away.
Any information about his would be greatly appreciated.
Because the German High Seas Fleet was not destroyed at Jutland.
After Jutland the German navy pursued a "Fleet in Being" stratagem. Essentially, by merely continuing to exist they threatened any attempted sea landing. This meant that any allied attempt to land troops behind the lines enmass had to account for the possibility that the entire remaining German battle-line might sally to stop them. That would mean drawing off a large number of ships from the blockade or the home fleet to provide an escort. This would of course weaken those forces increasing the chance of the German fleet escaping into open water again. And the possibility existed, however slim, that the German fleet could slip out and engage any transport fleet under cover of bad weather or some other favorable opportunity with horrendous casualties to the troops on transports.
In addition to what Vampire_Seraphin has said; the area in question with "tons of room around Bremen", the Heligoland Bight, was heavily mined by both sides and has extremely tight navigation channels. Amphibious landings would have limited options as to where when and how it would be possible for them to come ashore. Amphibious landing technology was not as advanced as it became by 1944.
Pre-war the Germans had also constructed the Kiel Canal, which connected the Baltic and North seas. This Canal would have allowed their fleet to sail out of the Estuary of the Elbe and directly attack any Invasion force.
They did try to 'Flank' again after Gallipoli, in what became known as the "Macedonian Front" or the Birdcage was an Allied expeditionary force which landed to support Serbia in 1915, but ended up trapped in and around Salonika (Thessaloniki) which ended up as a drain on men and material for the rest of the war.
I would recommend Castles of Steel: Britain, Germany, and the Winning of the Great War at Sea and Dreadnought by Robert K. Massie.
Strictly on amphibious plans of action against Germany that covers the North and Baltic Sea, the predecessors of the plans during WWI goes back to 1904-1905, defined mostly by the Moroccon Crisis one of the reasons that strengthened the Triple Entente. These were formed mostly to divert German forces and relieve the French forces inland in a possible war scenario.
Of these, Wilson's proposal for an amphibious attack on Schleswig-Holstein in 1905 was a plan to carry 60000 British troops "through the labyrinth of sandbanks and shoals shielding the Gennan seaboard to a landing in Schleswig-Holstein" wasn't regarded as impractical at first but later it was ruled out because of the overwhelming strength of the German forces in the region.
Seizing Borkum was also projected both pre-war and during war, especially by Churchill as Borkum was his main focus for a Baltic plan, it would serve as an advanced base to control Ems-Jade. Similar plans for Wangeroog and Heligoland (again by Wilson) as a forward base were raised but they were already dismissed in Loch Ewe conference at the beginning of the war and they didn't find more support. Just like Churchill, Admiral Fisher was a long-time advocate of Baltic Project, a landing in Schleswig-Holstein or Pomerania, especially since he saw the Army as a projectile to be used in amphibious plans but despite sharing a common Baltic offensive, Churchill and Fisher never reached an agreement which was also mentioned in the World Crisis after Fisher's death.
Of course one of the reasons why these plans were made long before the war was that especially after the Moroccon Crisis, a war with Germany was seen as inevitable especially by Admiral Fisher.
It's safe to count plenty of reasons why these plans were never materalized, I guess.
First, arguably, the Army was in favour of pursuing a continental war in their own strategy rather than being a part of a naval plan. Secondly, the Royal Navy projected, in order for a successful Baltic offensive, a force large enough, an Anglo-Russian expedition was needed but that had severe organizational difficulties and the availability of Russian troops were dubious at best.
Onto the more realistic reasons, rather than political or theoretical ones; because of the wide use of mines, German U-boats, narrow channels of the Belt with the German fleet already deployed there held a devastating risk for the Royal Navy as was proven in Dardanelles later in the war. It's good to point out that during WWI, it took a great deal of time to sweep mines and unlike WWII where the Allies managed to decipher German naval mine technology and its electromagneticity, the mines posed a greater threat despite Jellicoe's intense caution. Coupled with the U-boats' kleinkrieg strategy, right at the beginning of the war light cruisers like HMS Amphion and Pathfinder were lost along with the cruiser HMS Hawke, not to mention HMS Audacious, a dreadnaught, sunk by a mine.
There was only one plan that almost materialized and it was an offensive not in Baltic but on Belgian shore in 1917 which was called by Admiral Jellicoe because of the inability in attacking submarine pens in Bruge but the amphibious landing was to be followed by the victory of Third Ypres campaign, that never happened.
Further Reading:
War Machinery and High Policy : Defence Administration in Peacetime Britain, 1902-1914; 1973, Nicholas D'Ombrain.
The Papers of Admiral Sir John Fisher v.1-2, 1964, Ed. Lt. Cdr. P. K. Kemp
Strategy and War Planning in the British Navy, 1887-1918; 2012, Shawn T. Grimes
The Crisis of the Naval War, 1920, John Rushworth Jellicoe