The Israeli government today expressed "disappointment" with Lebanese President Amin Gemayel's public statements about the Israeli presence in Lebanon but suggested that Gemayel's public posture might conceal a more flexible private attitude.

In the first official reaction here to Gemayel's speech at the United Nations and the reports of his meetings with President Reagan and other U.S. officials, Israeli officials said they will stick to their positions despite Gemayel's objections.

Specifically, the Israelis said they were disappointed by Gemayel's demand that Israeli forces be the first to withdraw from Lebanon and his refusal not only to sign a peace treaty with Israel but to sign a formal agreement establishing "security arrangements" in southern Lebanon.

The net effect of Gemayel's visit to the United Nations and Washington was to put distance between the new Lebanese president and the Israelis. Last week, Israel's negotiating positions on a troop withdrawal from Lebanon was presented to the Lebanese government by David Kimche, the director-general of the Israeli Foreign Ministry, who reportedly came away convinced that these positions were acceptable to Lebanon.

Israel is demanding that the remaining Palestine Liberation Organization forces be the first to leave Lebanon, after which it would be willing to accept a simultaneous withdrawal of Israeli and Syrian troops. The Israelis have set as their goal a formal peace treaty with Lebanon but are demanding as a minimum an agreement establishing a "security zone" in southern Lebanon before their troops pull out.

Today, Israeli officials appeared still to hold the view that Gemayel could be persuaded to accept their demands in the right circumstances. They noted that there are several reasons for the Lebanese president to distance himself in public from Israel, including Lebanon's need for a massive infusion of financial aid from the Arab world to rebuild itself after years of warfare.

Moreover, the Israelis argued that Gemayel, who also wants the Syrians and Palestinians to withdraw from Lebanon, knows that his best hope of achieving that goal rests with the Israeli presence in southern Lebanon.

"He knows that if we leave, Lebanon will be back to where it was in June, before we drove the PLO out of southern Lebanon," one official said. "He knows that his only chance of getting the PLO and the Syrians out is by playing the Israeli card. Once we are out, he loses any leverage."

Against this backdrop, the official said, it is important to "distinguish between public statements and quiet diplomacy." While Israel was "disappointed," he said, "we still hope to achieve what we set out to achieve."

Overall, the reaction that Israeli officials provided to foreign correspondents was mild, suggesting that they believe that Gemayel's negotiating position remains flexible.

Other public figures in Israel were more critical of Gemayel. Opposition Labor Party leader Shimon Peres said he found Gemayel's public statements in the United States to be "shocking."

Peres said that by "turning his back on Israel" the Lebanese president was "making a great mistake."

Israeli radio reported tonight that Foreign Minister Yitzhak Shamir has been invited to meet again with Secretary of State George P. Shultz in Washington before returning to the Middle East.

The invitation, which Israeli officials had hoped would be forthcoming following Gemayel's visit to Washington, is expected to provide the Israeli government with a clearer picture of Gemayel's intentions and posture in the troop withdrawal negotiations.