The international airport on Grenada, denounced by President Reagan as a strategic threat when it was being built by Cubans, has been finished by Americans and now is being hailed by U.S. officials as an asset for tourism.

The 9,000-foot facility, which formally opens Sunday, has become the most visible sign of an extensive U.S. commitment here since U.S. military forces invaded the island a year ago Thursday and shipped out Cubans with their hands tied behind their backs. The Agency for International Development is spending $19 million to finish the project, begun in 1980 by the Marxist-oriented government of the late prime minister Maurice Bishop.

Peter McPherson, the AID director, has announced plans to attend opening ceremonies for the $70 million facility, called Point Salines International Airport. For the first time, jet passenger planes will be able to fly directly to Grenada, eliminating tourists' need for overnight stops to change planes in Barbados or Trinidad.

"We've wanted it for 20-odd years, and now we've got it," said Geoffrey Thompson, a St. George's businessman. "What are we going to do with it? We're going to make some dollars with it. And what are you going to do about it? You're going to make sure no one mashes it up for us."

The airport is the largest portion of an aid program totaling $57.2 million, more than half of Grenada's gross domestic product. Despite the program's relative size, however, Grenadians have begun to complain that so far it has produced no visible effects on the economy.

"People are beginning to get bitter," said George Brizan, a candidate in the Dec. 3 parliamentary elections and a leader of the U.S.-Backed New National Party.

Unemployment has remained at an official 30 percent, for example, about where it jumped after U.S. and Caribbean invasion forces disbanded the People's Revolutionary Army and put its 1,000 members into the job market. A rush of U.S.-financed road repairs soon after the invasion has died down, they point out, and potholes are nearly as bad as they were under the flagging Bishop revolution.

"I've been looking, but I haven't seen anything," said Robert Grant, a lawyer and independent political candidate. "Maybe they're waiting until the airport is finished so the planes can land and bring in those green dollars."

Ambassador Loren Lawrence, who heads the 27-person U.S. diplomatic mission here, said emphasis on preparing for infrastructure improvements and straightforward funding of the interim government's balance of payments and operating budget has meant most U.S. aid has gone unnoticed so far.

"A great deal has happened, but it's not visible to John Q. Public," said Lawrence.

U.S. officials pointed out, however, that a $6 million project to begin as soon as airport construction ends will create 2,500 jobs for about four months. This will include road and bridge repair and improvements in electricity and waste removal, they said.

U.S. private investment, which the Reagan administration sought to stimulate, also has been slow to materialize. Although the interim administration has approved 26 such proposals, only two American projects are in operation.

One, a wooden-toy factory, employs 40 persons and has invested about half a million dollars, a foreign business analyst said. A spice-packaging enterprise employs a dozen Grenadians on an investment of about $100,000, he estimated.

Most businessmen are hesitating until the elections, according to Nicholas Brathwaite, chairman of the interim government installed by U.S. forces after the invasion. Return of Eric Gairy to power, with the potential for unrest he would bring, could cause investors to change their minds, he added.

The political situation also will help determine how long the 250 U.S. Army troops here remain, Brathwaite said. They are scheduled to be phased out next year as the Grenadian police near the planned force of 650.

About 430 Grenadians have been recruited but only half have been trained, U.S. officials said. So for the moment, U.S. military police help man the island's six main police stations and regularly maintain a presence in another five, said Lt. Col. Arthur Graves, who commands the Ft. Bragg-based unit.

The military police, along with a 20-man Security Assistance Command Team and seven marine guards for the embassy, are all that remain of an invasion force that at its peak included 11,000 men on land and sea, equivalent to more than 12 percent of Grenada's population.

Capt. Olin Saunders, a spokesman for the U.S. troops, said Grenadians have kept up the warm welcome they gave U.S. soldiers during the invasion a year ago. Most patrols are still greeted with waves and smiles, he added, and two soldiers have married Grenadian women.

The atmosphere has endured despite the accidental killing of a 13-year-old Grenadian by an American soldier Aug. 21 in a police station. The 20-year-old private was cleaning his .45-caliber sidearm when it went off, fatally wounding the youth, Graves reported. The soldier was returned to the United States, and charged with failing to follow safety procedures.