[6][7][3] In April 1918, during World War I, Gibson was commissioned a Lieutenant in the 137th Infantry of the American Expeditionary Force, and later that year was posted to the front.

In 1922, Gibson moved to Los Angeles, establishing a practice, teaching at Southwestern Law School, and becoming active in Democratic politics.

He served as a campaign advisor to Culbert Olson in 1938, and in December of that year the governor rewarded him with the position of Director of Finance.

[12][13] Then, in August 1939, Olson appointed Gibson as an associate justice to the Supreme Court of California to fill the vacant seat of William Langdon.

[14][15] Among Gibson's notable cases as an associate justice was his dissent from a contempt citation for commenting on judicial proceedings.

[18] In June 1940, Chief Justice William H. Waste died in office, and Governor Olson appointed Gibson to the position.

Gibson's notable cases as Chief Justice include his 1944 opinion in Ybarra v. Spangard concerning the negligence doctrine of Res ipsa loquitur in torts.