The producers of Shady Grove Music Fair in Gaithersburg and its sister facility near Baltimore have decided to close both enterprises, which they say just can't compete any longer."

Some productions may contued at Shady Grove and at Painters Mill Music Fair in Owings Mill this summer, but the two unsubsidized commercial theaters are unable to turn a profit under current conditions, according to co-producer Sheelly Gross, who was reached in Philadelphia yesterday.

Gross said that Shady Grove, a 5,000-seat theater-in-the-round that opened in 1962, has been "suffering very heavy losses over the past three years."

He attributed the closings to intense competition from the Kennedy Center and Wolf Trap Farm Park for the Performing Arts, both non-profit organizations.

"Shady Grove and the dinner theaters," said Gross, "are literally the only live commercial entertainment enterprise in the Washington area. Though we wer originally highly successful, we just can't compete any longer. The Baltimore operation will have some attractions this summer, but it will be phased out of our future plans.

"Our lease on Shady Grove," Gross continued, "expires July 31, 1979. We may do occasional things now and then, but costs have virtually quadrupled over the years. Our losses have had to be made by our successful operations in the Philadelphia and New York areas."

Gross and his partner, Lee Guber, will operate the Jones Beach Amphitheater just outside New York City this summer.

Gross said yesterday that he doesn't see Shady Grove continuing as a theater after his lease expires. He said there has been informal talks of turning the property, once controlled by realtor Sam Eig, into a shopping center.

Guber and Gross, whose theaters stretched from New England to Florida, now are more active in producing for New York. Their revival of "The King and I," starring Yul Brynner, has just grossed its biggest week and in the course of 11 months will have grossed $2 1/4 million on a run which was considered iffy when it began.

Opened as a tent theater, with Olney Summer Theater as its major competition, Shady Grove initially was very successful with big-name stars in popular muscial shows of current or recent status.

In its fifth season it was converted to hard top with plans for year-round use. With the exception of two popular musicals, "You're a Good Man, Charlie Brown" and "Dames at Sea," the year-round programs didn't pan out.

In recent years the policy was altered in favor of variety, including popular-music bands and stars, with the musicals less in evidence.

A factor in the reduction of musical plays was the insistence of the musical shows' publishers that their percentage of the gross include the parking fee, which was free for Shady Grove customers but which the publishers insisted was part of the ticket price.