That isn't what he said. He said, "If I could do what I wanted, a synagogue would also be established on the Temple Mount."
And obviously that is not something he can do.
Now, reasonable people can argue whether he was impolitic in saying this, or whether it is a stupid thing to say, Ben Gvir is being pilloried as much in the Israeli media as in the Arabic media.
But Ben-Gvir is correct. Jews should have equal rights on the Temple Mount, including prayer, including even building a synagogue.
Why would any reasonable person disagree with equal rights? They can say that it is not a good idea practically, but no one can say that Jews have no historic or religious or legal claim to the Temple Mount, the holiest site in Judaism.
All the other issues - politics, security, public relations - exist and are real, but they are all secondary to human rights. The right for Jews to access and pray at their holiest spot is a human right.
The "status quo" is inherently a violation of Jewish human rights.
If it was up to me, yes, I would build a synagogue there. Perhaps in the large plaza on the southeast of the Mount, where one hardly ever sees Muslims (except playing soccer.) This is part of the Mount that was not a part of the original site, only built as an extension in Herodian times, and there is no possible way it is on the site of the original Temples which would be forbidden under Jewish law.
There are also solid reasons why building a beautiful, large synagogue there would be good for Israel and for the region.
I have no problem with the government of Israel promising to uphold the status quo for temporary political reasons, but it should make it clear but we will never concede the rights of Jews to ascend and worship there.