Retiring Montgomery County Executive James P. Gleason informed incoming executive Charles W. Gilchrist yesterday that he has imposed an immediate hiring freeze to help alleviate some of the financial problem Gilchrist will inherit.

Gleason, Gilchrist and their chief aides spend 90 minutes behind closed doors discussing a lengthy transition agenda of crucial policy decisions now pending, ranging from budgets and landfills to housekeeping matters like keys to administrative offices.

By instituting the hiring freeze, which will prevent filling of about 200 vacancies, Gleason went a step further than the County Council, which recently requested a partial hiring freeze to generate some surplus for the 1980 budget.

Budget officers have projected a $21 million deficit - based on current tax rates - if existing programs are simply continued in 1980. The projection is based on a decline in anticipated revenues and increased debt service and Metro subsidies.

"He will have a heck of a tough time handing the problems of government," Gleason said of Gilchrist, "and I want to help him as much as I can." Gilchrist, who pledged during his campaign to "level off" government growth, said he concurred with Gleason's decision on hiring.

The, with the jocularity that has become characteristic of the waning days of his eight-year term, Gleason quipped: "After all, how can you treat a man who is going into the desert?"

Gilchrist began his preparations for the executive office in the same low-key style with which he ran his successful campaign against Republican Richmond M. Keeney, a planning commissioner.

He also was surrounded by the key aides that helped him to victory - campaign manager Jack Sexton; Tom Stone, assistant chief of park planning at the Montgomery County Planning Board, and George Spring, a scientist and a campaign aide to such politicians as Sen. Paul Sarbanes (D-Md.).

Gleason's staff started planning for changeover six weeks ago when Lewis T. Roberts, an assistant adminstrative office, drew up an eight-page agenda for the victor.

When Gilchrist and his staff enter their new jobs Dec. 4, they will have been briefed on everything from fiscal issues to employe morate, intergovernmental relations and, in management parlance, "processes for policy development, decision-making and implementation."

When they have digested the Roberts memo, they should know how to get "on the payroll," find "keys to office, car, desk, restroom, elevator," operate "elevator locks" and handle emergencies or disruptive persons on the 7th floor" where the executive has his offices.

"There are so many housekeeping matters," Lucille Harrigan, a council staff member said yesterday as she pored over briefing papers for four new council members.

"The most urgent things we need to know are what names they want on their name plates so they can be in place in time she said.

Meanwhile, Gilchrist and his small entourage meandered two blocks from the county office building to their temporary quarter - a two-room sulte in a freshly painted government annex where visitors are directed through halls by bold blue supergraphics on white walls.

There in office equipped with a desk, conference table, a framed copy of the Montgomery County seal and four volumes of the county code, Gilchrist will go to work Monday, walking the three quarters of a mile from his Rockville home.