On Friday, the last day polling is allowed, Gantz's centrist Blue and White party was projected to take 30 seats, more than the 26 forecast for Netanyahu's right-wing Likud, according to a poll in Yedioth Ahronoth newspaper.
Israel and Palestine have had border and sovereignty disputes for several decades.
Netanyahu, asked why he had not declared Israeli sovereignty over large West Bank settlements as Israel has already done in the occupied Golan Heights and East Jerusalem, said he was already discussing the move.
The West Bank is home to about 2.8 million Palestinians and more than 400,000 Jewish settlers.
"I said there shouldn't be the removal of even one settlement" from the occupied West Bank, Netanyahu told Israel's Channel 13 television.
What happens to the land is one of the most contentious issues between Israelis and Palestinians, who argue that the presence of settlements would make a future independent state impossible.
Senior Palestinian official Saeb Erekat said Netanyahu's statement on annexation was "not surprising".
Trump has teased the long-promised US peace proposal as the "deal of the century", but it will undoubtedly be as Netanyahu-sided as everything else the Trump administration has done when it comes to Israel.
In a Saturday interview on Channel 12 television, Netanyahu said beyond keeping settlements in place, he was planning "the next phase" if he is re-elected - annexing settlements.
"You are asking whether we are moving on to the next stage - the answer is yes, we will move to the next stage", he said. He criticized Palestinian Authority President Mahmoud Abbas as someone who "has also not been faithful in pursuing peace", pointing to inflammatory rhetoric and his "inability to bring his side to the table to negotiate in good faith". "Settlements are illegal and they will be removed".
What is the political background?
Many analysts see the timing of his comments on annexation as an effort to shore up support among voters on the right who might be tempted to support numerous smaller parties that are considered potential coalition partners with Likud.
In Israel's system of proportional representation, the party with the most support still needs other parties to gain a majority in parliament and to form a government.
In Mr Netanyahu's own Likud party, 28 out of the 29 lawmakers running for re-election are on record as supporting this approach. In the video posted on social media sites, the prime minister asked voters to "Return to the Likud".
He also told Trump that any U.S. reconciliation plan for Israel and Palestine must not include the removal of the settlements from the occupied Palestinian lands. In his first year in office, Trump had also recognized Jerusalem - the disputed city claimed as capital by both Israeli and Palestinian people - as Israel's capital, breaking with decades of USA foreign policy. In response Mr Abbas cut off relations with the USA, saying the United States could no longer be a peace broker.
Mr Netanyahu mentioned the recent U.S. recognition of Israeli sovereignty over the occupied Golan Heights as an achievement.