He was a Ministerialist member of the Western Australian Legislative Assembly from 1897 to 1911, representing the electorates of North Coolgardie (1897-1901) and Menzies (1901-1911).

He rose to become Treasurer from 1910 to 1911, a role that also entailed him acting as Premier if Wilson was absent, but lost his seat at the 1911 state election.

[2][3][4][5][6][7][8][9][10][11] In 1897, he was elected to the Western Australian Legislative Assembly as the member for the new seat of North Coolgardie, narrowly defeating future federal MP Hugh Mahon in a bitter contest, and transferred to the Menzies electorate in 1901.

[4] In 1904, while Minister for Mines, Kalgoorlie tabloid newspaper The Sun made allegations about Gregory's handling of a mining lease dispute, leading to the landmark imprisonment of its editor, John Drayton under parliamentary privilege after Drayton refused to appear before a parliamentary committee into the incident.

[14] He attempted to re-enter parliament at a by-election for the Metropolitan Suburban electorate of the Legislative Council the following month, but was again defeated.

[15][2] Following the loss of his state seat, he spent two years farming on a 6000-acre pastoral property 30 miles east of Wickepin.

[3][2] Gregory died at Mount St. Evin's Hospital in Melbourne in 1940, aged 80 and then the oldest member of the House of Representatives, following a short illness, although he was said to have been in indifferent health for several years.