The son of Reverend Nicholas Gilman (who died when Tristram was twelve years old),[2] and Mary Thing,[3] he graduated from Harvard College in 1757.

He was ordained as minister at the now-demolished "Old Ledge" meeting house, in the Broad Cove area of the town, on December 8, 1769.

[1] Shortly after taking the position, he became the original 1771 occupant of the Gilman Manse house at today's 463 Lafayette Street.

In his Catalogue of the First Church, Reverend David Shepley, the second minister of the church,[2] wrote of Gilman: "[He possesses] a vigorous physical frame, endowed with strong features in his mental constitution, studious, evangelical, ever diligent and enterprising in the duties of his calling, he soon attained uncommon ascendancy over the minds of his people, rose to eminence in the vicinity, and at his departure left his strong impress on the place blessed by his long-continued and successful labors and influence.

Gilman married Elizabeth Sayer (1747–1790), a native of Wells, Maine, around the time he began his ministry at the Old Ledge Church.