In May 1915, the light horse were ordered to Gallipoli dismounted to reinforce the infantry. Forsyth arrived at Anzac Cove on 5 May 1915. On 19 May 1915, he became the Assistant Adjutant and Quartermaster General (AA & QMG) of the 1st Division. From 30 May to 8 June he was acting commander of the 2nd Brigade in the absence of Colonel James Whiteside McCay, who had been wounded at Krithia, along with most of the brigade's officers. When McCay was evacuated with a broken leg, Forsyth became commander of the 2nd Brigade on 26 July 1915 and was promoted to colonel and temporary brigadier general. He remained at Anzac with his brigade, fighting another battle at the German Officers' Trench in August, until it was evacuated to Mudros on 10 December 1915.