Gaza has land borders with two countries: Israel and Egypt. Israel has partially closed its own borders with Gaza. It has no presence on Gaza’s border with Egypt. While Egypt has legal obligations to Israel and other states to prevent certain types of material and persons crossing into and out of Gaza, Egypt’s decision to completely seal the border with Gaza is its own and not attributable to Israel. For more on Egypt’s actions toward the Gaza border, see a blog entry on this web site from several weeks ago concerning subject.
Israel also patrols Gaza’s air and sea space, but this is a separate issue, which I addressed in the article you read. Please also see my answer to the question above concerning Israel’s right to block arms flowing into Gaza.
Finally, I should add that all this is irrelevant to the question of “collective punishment.” Even if Israel were in full control of Gaza’s land borders, this would not constitute collective punishment. Notwithstanding the many unfounded criticisms of Israel, I am not aware of any prosecution in modern legal history based upon the theory that economic sanctions constitute “collective punishment”