The Marxist Palestinian guerrilla leader leaned across the table and talked about the "new Lebanon" envisioned by the Christian Maronite leader Bashir Gemayel.

It would mean a new political system in which sectarianism would "gradually become secondary" and reforms would be made assuring a liberal democracy. That would not be so bad, he indicated.

Gemayel, he continued without blinking an eye, "does have a chance" of becoming the president of this "new Lebanon," particularly since he has promised the Saudis that he will cooperate with Lebanon's Moslem leaders in the search for a solution to the current crisis.

That a Marxist Palestinian could for one moment take seriously the idea that Gemayel, long the personification of the guerrillas' worst Lebanese enemy, might become president of Lebanon is one measure of the extent to which the Israeli invasion has turned this country's politics upside down.

The Palestinians and leftist Lebanese have heretofore regarded Gemayel as Israel's Trojan Horse in the Lebanese camp--the rightist Christian militia leader who was acting in league with Tel Aviv to push both the Syrians and the Palestinians out of the country.

Yet the Marxist guerrilla official was not the only Moslem in West Beirut last week taking Gemayel seriously. Others included Walid Jumblatt, Druze leader of the Lebanese leftist National Movement, and Saeb Salam, a spokesman for the moderate Sunni Moslem community.

Jumblatt, in a midweek meeting with reporters at his home, spoke cautiously but respectfully about the 34-year-old military leader of the Christian militia.

Furthermore, despite his deep political differences with Gemayel, he felt the Maronite chief had handled himself well on the now defunct National Salavation Council.

"Gemayel has a long-term policy in mind," Jumblatt remarked. "He is playing another card" than that of simply helping the Israelis crush the Palestinians. "He is playing the card of the presidency," he added.

Indeed, the next day Gemayel made public that he is a candidate for the presidency in the elections scheduled for September.

Meanwhile, Salam, a former prime minister and a key figure in the negotiations with the Palestinians, found time Thursday to arrange a telephone conversation between Gemayel and Palestine Liberation Organization leader Yasser Arafat.

"We have got to recover the Christians from the hands of the Israelis if we are going to find a national solution to our political problems," explained Salam.

That Arafat felt the necessity to speak with Gemayel was yet another indication of the Maronite leader's growing acceptability as a man who must be dealt with.

If three weeks of intense Israeli military pressure on this war-weary nation has done anything, it has been to shuffle the Lebanese pack of political cards in such a thorough and unexpected way that Gemayel is no longer dismissed out of hand by the Moslem and Palestinian leaders of West Beirut.

This is partly because he is now viewed as the man backed by the powerful Israeli military, which has a foothold in Gemayel's tightly run East Beirut.

But partly, too, Moslem leaders are giving him credit for an unexpected degree of statesmanship at a time when they fully expected him to throw in his lot, and his sizable militia force, with the Israeli invaders.

Instead, Gemayel has carefully postured himself to keep his public distance from the Israelis, repeatedly calling for the withdrawal of "all" foreign forces, including the Israeli one, from Lebanon. He has also demanded that the Lebanese Army be used to fill the vacuum and help restore the authority of the central Lebanese government across the entire country. This political platform is now endorsed not only by the Reagan administration and Saudi Arabia but also by a fairly broad spectrum of Moslems and Christians now bottled up in West Beirut who are fed up with the lawlessness that has prevailed here ever since the 1975-76 civil war.

It is not just altruism or high ideals that leads Gemayel to adopt such a platform nor is it new. The Maronite leader has everything to gain from it--personally and politically.

If the Syrians were squeezed out of Lebanon, Palestinian and leftist Moslem power reduced and the Lebanese Army left to fill the vacuum, Gemayel's Lebanese force would be likely to emerge as one of the strongest players in the new political game taking shape here.

For one thing, the 21,000-man Lebanese Army is widely regarded by the Moslem communities as having mostly Christian officers, although Army spokesmen deny this.

For another, Gemayel seems likely to have the full political backing of Saudi Arabia, the United States and France, provided he can learn how to live and cooperate with a wide enough spectrum of West Beirut's Moslem leaders.

This, in the eyes of his old Moslem and Palestinian foes as well as some outsiders who know him well, is no small "if."

For one thing, the Israelis, whose on-again off-again support for him has been a very mixed blessing, could easily destroy his dreams of one day leading a "new Lebanon."

The threatened Israeli assault on West Beirut, if it is carried out, is likely to spell disaster for Gemayel.

The Moslem communities--Shiite, Sunni and Druze of the left and right--almost certainly never would forgive him if there were hundreds of civilian casualties.

But even short of an Israeli invasion of West Beirut, Gemayel's close cooperation with the invaders could discredit him among Moslems.

There are signs that just such a process may be developing. At least, there is a growing conviction among Moslems here that this is happening.

Moslem members of the National Salvation Council are blaming Gemayel for the failure of the body to meet with Arafat late last week to discuss a way out of the present crisis, even though it was the Maronite leader's idea in the first place.

Salam, who tried to arrange the meeting, said Gemayel "completely disappeared" at the crucial moment and could not be contacted. As a result, the meeting was never held and the council went out of business.

Sunday, Salam told The Washington Post that he had "definite evidence" that Gemayel's Lebanese Forces and the Israelis were actively collaborating, and he complained of "looting and robbing" by the Christian militia in coastal towns south of Beirut where the Israelis had ousted the Syrians and Palestinians and the militia had taken over.

Salam said he had asked Gemayel Sunday about whether there was collaboration between the Israelis and his Christian forces, and Gemayel was "not so firm in his denial."

Moslem Lebanese leaders are concerned that Gemayel will be lured by the Israelis into ordering a more active role for his militia forces in the Israeli campaign to crush the Palestinians.

A number of Gemayel's top advisers have already taken a very tough stand against any compromise with Arafat, while others are openly advocating a "military solution," hopefully by the Israelis, to finish off the guerrillas.

Gemayel, who is still regarded as a man believing in military answers to political problems, so far has not publicly identified himself with this position. But, if the stalemate continues, the temptation to revert to force may well grow.

After all, it was by wiping out his most serious rivals that he became the undisputed chief of the Christian Maronites. His Christian militia's inclination to resort to force has already begun to reemerge in the mountainous Chouf area east of Beirut, where there are many reports of clashes between it and the Moslem Druze inhabitants. As the Syrians are thrown out of the area by the Israelis, Gemayel's militia is apparently muscling its way in to fill the vacuum.

Such a course of action now, say Moslem leaders and many analysts of Lebanese politics, could doom Gemayel's proclaimed hopes of a "national solution" in cooperation with the Moslems and thus also his hopes of becoming president.

Gemayel, who has by all accounts come a long way in the thorny politics of Lebanon since the civil war seven years ago, may still be very far from getting the power he seeks.