During WWII Gross accepted the role of an official war artist. In 1941, with a temporary commission of captain, Gross was attached to the 9th Army and painted within the Egyptian, Syrian, Palestinian, Kurdistan, Lebanese, and Mesopotamian theatres of war. In 1943 he transferred to India and Burma to witness the front line battle against the Japanese.
Yes, you are right. He received several proposals for (military) commissions.
A military commission is a commissioned officer's official document of appointment.
After he was officially appointed as Captain he went on to become an official war artist in those locations where he was appointed.