At ~8:30 in this video, Indy Neidell makes this claim, but I wasn’t able to find reference to it. Given that that could be one of the largest disasters in maritime history, I was surprised that it wasn’t able to be easily validated; to be fair, I only speak English so would obviously only be looking at English sources so that could be where I’m missing it.
Can anyone provide additional context here?
He seemed to have used the wrong sources (but you can't blame him). The British claimed the troop ship Gul Djemal was sunk by their submarine HMS E14 on May 1. And numbers vary from 1,600 to 4,000 on-board (based on Turkish-German sources respectively), while the British exaggerated and claimed 6,000, but this was not the case. The Captain of E14, Edward Courtney Boyle, was awarded the Victoria Cross on 21 May 1915.
In actuality, the ship was never sunk (it in fact survived the war and lasted until 1950), only damaged by a British torpedo and escaped to Constantinople, where it was repaired by German engineers. Meanwhile its crew was evacuated by ferry to Gallipoli.
German records state the ship was lightly damaged with no loss of life:
E 14 succeeded in an attack on the turkish transporter Guldschemal 5071 t which was on the way to the Dardanelles with troups on board and was hit on the front side. Contrary to english statement the ship did not sink.(a bounty of 31000 pounds was paid). The bow boke twice but the ship could be bought to Constatinople without human loss. Of course the ship could not be repaired during the fights of the Dardanelles.
Source: Der Krieg in den türkischen Gewässern / Hermann Lorey Bd. 2. Der Kampf um die Meerengen
Ottoman sources similarly deny anyone was killed, even providing a different date for the incident:
I hurried to the deck after him. Gül Cemal was hit just on the water line. Water was pouring inside the vessel. But the crew immediaely closed all the doors and kept the ship on water. The captain let the damage controlled and informed me that we could not move forth. The captain signaled Pathmos to contunie her route and informed her that he was returning to the nearest shore which was the island of Imrali. We disembarked Gül Cemal at Imrali without a casualty. The next day on May 11st 1915 a vessel of Şirket-i hayriye nr 67 and the Samsun came to Imrali. But they were little ships. I couldnt boarded all my regiment. At evening a sidewheer came to Imrali. On May 12 1915 we set foot at Akbaş on the shore of Gallipoli peninsula. As we came to Akbaş everybody cheered us. They thought that Gül Cemal was sunk and we all drowned
Source: Canakkale Hatıraları, Cemil Conk