Acting to prevent "rent gouging of a substantial number of tenants," Montgomery County Executive Charles W. Gilchrist yesterday proposed that a mandatory 10 percent ceiling be established to cover most of the county's 45,000 apartments.

Although he said he was "philosophically opposed" to government regulation of rents, Gilchrist said that at least 12 percent of the county's tenants have "suffered from excessive and unwarranted" increases since rent controls were lifted 14 months ago.

A spokesman for the county's apartment owners called Gilchrist's legislation a "return to rent control" and predicted it would lead to a rash of conversions to condominiums.

"I think it has struck the death knell for rental housing in the county" said Joseph Schuble, president of the Apartment and Office Building Association of metropolitan Washington.

The new Democratic executive also sent to the County Council yesterday a bill to consolidate the county's fire and rescue servie -- presently an assortment of 18 independent volunteer and paid departments -- into a new Department of Fire-Rescue Services.

Call the present system "antiquated," Gilchrist said he was concerned that the "inadequate chain of command, particularly at fire incidents," could create "public safety problems" for the couny's 590,000 residents.

Gilchrist recently ordered the investigation of a $500,000 apartment fire in Wheaton after a staff review, prompted by complaints from firefighters, found possible negligence and mismanagement in battling the blaze.

The two bills, the first legislation Gilchrist introduced since taking office Dec. 4, were presented as emergency measures "to try and improve, as rapidly as possible, situations that I found to be quite serious during the recent political campaings."

Gilchrist said his rent control measure would "put teeth" in the current voluntary rent guidelines. They were imposed by the County Council after seven years of rent control ended in December 1977.

During 1978, tow-thirds of the landlords held their rent increases to 6.1 percent, the maximum under the system of voluntary compliance. The exceptions, however, included rent increases of as much as 50 percent.

But the law gave the Office of Landlord-Tenant Affairs no authority to force rollbacks if the rent increases were found to be unjustified after they were investigated.

Gilchrist's bill would require that any increase above 7.6 percent for present tenants be investigated by the landlord-tenant office. Landlords who can prove that their increases are necessary because of higher costs would be permmitted to raise rents more than 7.6 percent, but no more than 10 percent.

The landlord-tenant office would be empowered to roll back unjustified increases to 7.6 percent under the proposed law.

The law would not affect vacant units unless the apartments become empty after rent increases above 10 percent or a landlord forces a tenant out "without good cause." Gilchrist recommended the law take effect through Jannary 1981.

"I do not philosophically endorse regulation of rents," he said. "I think it's long-range bad economies. It should not be in effect one day longer than it is needed. It certainly will not aid in getting investment in rental housing in this county."

Gilchrist's proposal received a strongly negative reaction from property owners and only a lukewarm response from tenant leaders yesterday.

"He's trying to be a hero with the taxpayers by presenting a 34-cent property tax cut in his budget," said Schuble, the apartment industry spokesman "but he's saying to us, 'tough luck, you guys are going to bear the brunt of subsidizing lowincome people in the county.'"

"By alleviating the problems in the worst places, Gilchrist has tried to meet us halfway," said tenant leader Phil Ochs. But tenant groups still prefer pending legislation by council member Elizabeth Scull to limit recent increases to 7 percent, he said.

The fire legislation calls for a single fire tax rate -- to reduce inequities, Gilchrist said -- and uniform training standards and equipment for volunteer and career firefighters.

Gilchrist proposed $313,000 in new fringe benefits for volunteer firefighters, who have declined from a majority to slightly less than half the county's 969 active firemen in the past decade.

Volunteers traditionally have prized their independence and have balked at efforts to centralize control of the fire service. But Walter Gold, a spokesman for the volunteers said yesterday that he anticipates no massive opposition "unless we are treated like second-class citizens under the reorganization.