First, while there has been no final agreement on Gaza's borders, the various rounds of the peace process have established what the de facto borders are. There's no real mystery there.
Second, to say that Gaza is not under siege because Israel withdrew is really disingenuous. Gaza has no control over its own borders and Israel and Egypt keep very tight control over everything that comes in and out, in the name of preventing military shipments. This is why there is so much deprivation.
Interestingly, Hamas is now saying it will agree a ceasefire based on this very issue -- they want open borders. From the AP:
"The Islamists view the current round of fighting as an opportunity to pry open the borders of Gaza, which slammed shut in 2007, after Hamas wrested control of the territory from its political rival, Palestinian President Mahmoud Abbas. In response to the takeover, Israel and Egypt ? then under Morsi's pro-Western predecessor Hosni Mubarak ? sealed off Gaza to disrupt Hamas rule...
Both Israel and Egypt's new leader have eased access to the territory since 2007, but many restrictions remain. But even Morsi ? who is sympathetic to Hamas as a fellow member of the region-wide Muslim Brotherhood ? has resisted Hamas calls for open trade between Gaza and Egypt. Morsi fears such ties could undercut attempts to set up a Palestinian state in Gaza and the West Bank, the territory on the other side of Israel, where Abbas has partial control..."