The German contributions to the fighting at Gallipoli mostly consisted of:

1. German officers and advisors in the Ottoman army. Key among them was Otto Liman von Sanders, commander of the Ottoman 5th Army, which was defending Gallipoli. As the senior Ottoman commander in the theatre, Liman von Sanders was influential and important, and had a commander's responsibility for the victory. Other German officers commanded divisions and regiments. Not all were German-commanded - of the two corps in the 5th Army, the commanders were German and Turkish (one each), and there were Turkish divisional commanders (famously, the 19th Division was commanded by Mustafa Kemal, later known as Mustafa Kemal Atatürk). German officers also held many staff positions.

2. Ottoman navy. The transfer of Goeben and Breslau to the Ottomans brought their German crews into the Ottoman navy, and the German commander of the squadron (i.e., Goeben and Breslau) became the commander of the Ottoman navy. Goeben, renamed Yavuz opposed the landings in April, but with little effect, being unable to stand up to the British battleships, and was forced to flee. Her marines had more impact on the fighting, being deployed on land as machine gun teams.

3. German navy. The U-boat U-21 sank the battleships Triumph and Majestic in late May, which reduced British naval artillery support for the ground troops by (a) sinking two major warships and (b) the temporary withdrawal of the rest of the fleet to avoid further losses.

4. Air force. The Ottoman air force was German-commanded, German-supplied, and many pilots and technicians were German. They made important contributions to reconnaissance.

5. A volunteer German pioneer (i.e., engineer) company. The soldiers travelled as independent volunteers from Germany to Turkey posing as civilians, arriving in June 1915. The original strength was about 200 men; most promptly became sick, and half the company was sent back to Germany due to health. Their strength was reduced to about 40 fit men, until further reinforcements arrived. Their role was as technical experts to help with mining and counter-mining (i.e., digging tunnels under enemy positions and filling them with explosives, and stopping the same being done by the enemy).

Mention should also be made of the Austro-Hungarian contribution: two artillery batteries, which arrived in November and December 1915, too late to have much impact.

As the commander of the 5th Army, Liman von Sanders certainly has some responsibility for the victory (and responsibility for the losses, too - he has been severely criticised by some Turks for having supposedly caused excessively high losses, being more interested in keeping enemy troops away from the Western Front than winning quickly and cheaply). However, he wasn't a military superman performing brilliant feats that other good commanders couldn't have done. Notably, successful key decisions in the initial defence were made by Mustafa Kemal and his Turkish corps commander, Mehmed Esad Pasha. Esad Pasha, as the local corps commander had more immediate responsibility for the defence than Liman von Sanders, and commanded well and effectively. (Liman von Sanders also expected the landings to be in the east of the Gulf of Saros, which impaired the immediate defence - his initial contribution was not positive.) Liman von Sanders made an important contribution to improving the quality of Ottoman leadership: commanders who failed to demonstrate competence were replaced, whether German or Turkish (something that perhaps the Entente commander, Hamilton, should have done).

The German marines, pioneers, and the air force made useful contributions. The marines were present from the beginning, but were few in number - their contribution was useful locally, but probably not critical to victory. The pioneers arrived after the initial landings had been contained, and after the disastrous Ottoman offensive to throw the enemy into the sea in mid May had failed (the sinking of Triumph and Majestic helped the Ottomans in the aftermath of the failed offensive, but this wasn't critical to victory).

Thus, it can't be truthfully said that the Germans made no contribution at all. Apart from the contributions mentioned above, the German staff officers helped improve the standard of staff work, which surely helped. On the other hand, there doesn't appear to be a convincing argument that the German contributions were essential to the Ottoman victory. While Liman von Sanders can be given the commander's responsibility for the victory, the man-on-the-spot was Esad Pasha, who may well have been able to provide a victory for a different commander of the 5th Army, whether German or Turkish.