From 1961 to 1964 he was on loan from the Port Authority as the first Executive Director of the Tri-State Transportation Committee.  He then returned to the Port Authority and in 1975 was named Assistant Executive Director.  In April, 1977 he was named Acting Executive Director.  He retired in this capacity in August, 1977 after four decades of public service.

After retirement Mr. Gilman was a special consultant to Cook College, Rutgers University and was a Senior Visiting Fellow in Princeton University's Department of Civil Engineering.

Mr. Gilman received the Port Authority's Distinguished Service Medal in 1962 and its Howard S. Cullman Distinguished Service Medal in 1968.  In 1977 he was named Metropolitan Civil Engineer of the Year by the Metropolitan Section of the American Society of Civil Engineers.  From 1968 to 1969 Mr. Gilman was President of the American Association of Port Authorities and has long been active in ASCE, the Transportation Research Board and ITE in addition to many commerce-oriented activities in the New York City area.