After joining the Belgian Army in 1910, Gilliaert received an officer's commission in July 1914.

With the outbreak of World War I, he fought in battles along the Yser River, near Ostende.

Gilliaert returned to Belgium in 1919, commanding units at home and in occupied Germany.

[1] After the successful conclusion of the campaign in East Africa, a part of the Force Publique was re-designated the 1st Belgian Congo Brigade Group and served in a garrison and rear-area security role in Cairo, Egypt and in British Palestine from 1943 to 1944.

[3] Gilliaert was made commander of the Force Publique in July 1944 and promoted to lieutenant-general in October 1951.