Netanyahu (or at least, all Netanyahu's defense policies that the left doesn't like) are already synonymous with Israel. If Gantz was Priem Minister, he would be doing mostly the same stuff and the non-Jewish left would loathe him as much as they loathe Bibi. They don't hate Bibi because he's a conservative, they hate him because he's a nationalist and every Israeli PM is going to be a nationalist.
There's no reconciling leftist internationalism with Zionism. Jews who are advocate leftist internationalism are not allies of the Israeli state and there's no reason for the Jewish state to care what they think.
It's not only the defense policy: it's the apartheid. I concur that both Ganz and Netanyahu are nationalists, but possibly they are nationalists of a different kind. Netanyahu is an unashamed ethno-nationalist who despises the concept of "peace" and seeks -step by step- a reactionary agenda. I think it's clear the State of Israel is following a path that it's increasingly authoritarian. Gantz is a military man and obviously a nationalist ("Blue and White"). However Gantz was still talking about "peace", albeit in very vague terms. Correct me if I'm wrong, but I recall Gantz stated something like: "Jews and Palestinians are here to stay, so we need an 'arrangement'". Netanyahu, on the other hand, is crystal clear: he doesn't want peace or "arrangements", only the permanence of status quo (occupation, apartheid) and the progressive annexation of the West Bank. Gantz is far from being an ideal counterpart, but I think it would have been better as PM (for Israelis and Palestinians, not only for the leftist gentiles). But maybe not much better; admittedly I was rather skeptic about him and his chances of beating Bibi at elections. Gantz is an exponent of the militarization of the Israeli politics. Netanyahu represents a step further (way ahead of revisionist Zionism, extreme degradation) in that process of militarization, as he is the exponent of more authoritarian and ethnic form of nationalism. It's not a coincidence that the best friends of Bibi are Trump, Putin and Bolsonaro. As you say in the last paragraph, the reactionary ultranationalists that govern the State of Israel don't need and don't care about the Jewish leftists and liberals. They have a different vision and worldview, clearly illiberal and regressive.