Lebanese President Amin Gemayel yesterday called on Americans of Lebanese descent to support his efforts to get all foreign troops out of their homeland, pledging to "restore the territorial integrity of our sacred soil" and "recover every inch of the land."

Gemayel, speaking to a large gathering at the Washington Convention Center, expressed "profound gratitude" to President Reagan and other Americans, especially the 17 who died when a bomb exploded at the U.S. Embassy in Beirut in April, for their efforts on behalf of Lebanon.

But the 40-year-old leader, welcomed formally by D.C. Mayor Marion Barry, warned that "the credibility of America" will depend upon the "success of the American commitment to Lebanon" and its ability "to effect the complete withdrawal of foreign troops from our soil."

Gemayel, whose speech was broadcast live on Lebanese television and radio, was repeatedly interrupted by applause from the enthusiastic crowd, some members of which came from other cities on the East Coast and Canada. The invitations were issued by the Lebanese Embassy, which estimated the attendance at 4,000 people.

His meeting here with members of the Lebanese community appeared aimed at generating increased lobbying of U.S. officials by Lebanese-Americans, who make up two-thirds of Arab-Americans, on behalf of the beleaguered government in Lebanon. Gemayel has scheduled similar meetings in Houston and Detroit.

In his introduction of Gemayel, Lebanon's ambassador to the United States, Abdallah Bouhabib, said, "We are here to seek your political support: Contact your representatives, your senators and tell them the cause of Lebanon; support President Reagan's initiatives."

"Fellow countrymen, I offer you my hand, on behalf of your dear brethren in the homeland; take it, I urge you, and each and every one of you give me your all, just as your brothers at home gave you of their all--for Lebanon is just as much yours as it is ours, and never will it pass out of Lebanese hands," Gemayel said.

" Gemayel's trip itself is pivotal for Lebanon," said Robert Basil, spokesman for the American Lebanese League, one of the largest American-Lebanese groups. "Lebanon is in danger of permanent partition . . . the Lebanese community is very, very concerned about the situation."

The Christian Lebanese leader, whose task of negotiating the withdrawal of Syrian and Israeli troops from his country is complicated by sectarian and religious strife among Lebanon's Moslem and Christian groups, made a point of having Lebanese Prime Minister Shafiq Wazzan, a Moslem, also address the audience.

Although Gemayel enjoys almost total support among Lebanese-Americans of Christian background, those who are Moslem have reservations about his ability to protect Moslem minorities from armed Christian militias outside the control of the army. This concern prompted members of the American Druze Society, relatives of the Druze sect in Lebanon, to ask for a private meeting with Gemayel during his Washington stay, according to the society's president, Sami Merhi.

During that meeting Friday, the Druze delegation urged Gemayel to disarm the militias and get a political accord between the Christian and Druze communities that have been fighting in the Chouf region of Lebanon before he sends the Lebanese army there, Merhi said. He added that he was encouraged by Gemayel's receptivity and "endorsement" of their suggestions.

The convention center meeting concluded a five-day Washington visit by Gemayel, who conferred with Reagan and high-ranking American officials during his stay. He left the city yesterday for Houston, where American relatives of the Lebanese leader are planning a family reunion. Organizer Edward Jamail, a distant cousin who said the spelling of the family name was changed by U.S. immigration officials in 1904, said he was expecting about 330 members of the Jamail clan to attend the reunion.