Born 4 August 1892 at Cookham, Berkshire, he was the brother of Sir Stanley Spencer. He studied at Camberwell School, and the Royal Collage Art between 1911 and 1912, where he took to wood carving. Between 1913–15 he followed his brother Stanley to study at the Slade School under the tutelage of Frederick Brown and Henry Tonks.
In 1915 he served a four year term with the Royal Army Medical Corps in Salonika and the Eastern Mediterranean. His experiences here would go on to influence his future work as an artist. At the age of 30 he met Sydney Carline, the brother of Hilda Carline, future wife of Stanley, and the Ruskin master of drawing at Oxford, Gilbert to joined the staff.
A country man at heart, Spencer’s work was highly informed by his upbringing in Cookham. His paintings were created in a stylised way in which the composition is designed elevate the content from village life to a more universal status, with much of his paintings focusing on the continuity of tradition and the enduring bond between man and nature.
Teaching kept Gilbert in comfortable employ until he was able to support himself as an artist, though he never stopped teaching. From 1932 to 1948 Spencer was Professor of Painting at the RCA , the Head of the Department of Painting at Glasgow School of Art between 1948–50 and, from 1950-7, the Head of Painting at Camberwell School of Arts and Crafts.