Born in Maida Vale, Jack was the son of Moses Gaster, the leader of the Sephardic Jewish Congregation in London, and Lucy Friedlander.
He studied at the London School of Economics and then entered a legal career, qualifying as a solicitor in 1931, and soon thereafter forming a socialist law practice with Richard Turner.
However, he was a champion of unity with the Communist Party of Great Britain (CPGB), and to this end was a founder of the ILP's Revolutionary Policy Committee.
The group produced a report covering a range of issues, including the conditions for prisoners of war, but were heavily criticised for their allegations of germ warfare by the United States.
[1] Gaster remained a prominent communist solicitor until he retired in 1990, and an activist in the CPGB until it was disbanded in 1991, a decision with which he strongly disagreed.