Ypres

Ypres was a town in France. During World War I, it was the site of a battle between the British and German armies "twentyodd years" before 1940 as Colonel Schott remembered. The British forces used mustard gas during the attack, inflicting heavy casualties on the Germans. (PROSE: Illegal Alien)

Colonel Gaunt was a veteran of Ypres. (PROSE: It's a Lovely Day Tomorrow)

Behind the scenes
There were in fact a total of five battles fought at or near Ypres over the course of the First World War. Which one Illegal Alien is referring to, or indeed if there was more than one battle fought at Ypres in the DWU, is not stated. The Battle of Passchendaele, referenced in the novel Byzantium! and the audio story The Next Life, is also known as the Third Battle of Ypres as Passchendaele is a town near Ypres, but this connection is yet to be made in Doctor Who fiction.