He was then invited to deliver twelve lectures before the Lowell Institute, Boston, after which he went to Europe on a tour of professional observation.

On his return to Boston, he advocated filling in the Back Bay district, urging this plan for years before his views were carried out by the state.

Commonwealth Avenue, now one of the finest streets in the world, is due almost entirely to his persistent efforts, along with Frederick Law Olmsted.

Gilman designed the H. H. Hunnewell house (1851) in Wellesley (then West Needham)[2] and, with Bryant, the Old City Hall in Boston (1862–65).

In 1865, he moved to New York City, where he designed the original Equitable Insurance Company's building, the Bennett Building for The New York Herald,[3] and St. John's Church and parsonage[4] circa 1869[5] in Clifton, Staten Island.