Gray steps into the role having been head of general expenditure for the Treasury, and following Field’s departure in September for a new role at the International Monetary Fund in Washington.
PRWeek also understands that Susie Squire, who currently runs the press operation for the Conservatives, will replace Cameron’s press secretary Gabby Bertin.
Gray will start after the Chancellor's Autumn Statement, while Squire will move into the role shortly after Bertin goes on maternity leave on 15 November.
The moves come as the Government looks to regain the initiative after a difficult summer.
The loss of Field and Bertin as trusted media advisers to Cameron followed the departure of ex-News of the World editor Andy Coulson.
Gray will work alongside the team of special advisers under Cameron's director of communications Craig Oliver.
However, his position will be within a civil service capacity and he will report to the PM's principal private secretary, Chris Martin.
The 36-year-old previously held a range of posts within the Treasury, including private secretary to the Chancellor and head of press.
Squire’s move follows just four months as head of press for the party, a role she was given after two years as a spad for Work and Pensions Secretary Iain Duncan Smith. Before this, she rose to prominence as political director for the TaxPayers' Alliance.
Bertin is known as a well trusted and longstanding adviser, with one PRWeek source recently claiming the ‘hole she will create is impossible to fill.’