After Hamas took control in Gaza in June 2007, the existing Israeli policy of closure was tightened to a blockade restricting the entry of food, fuel, and other basic goods....
As the occupying power, Israel bears the foremost responsibility for ensuring the welfare of the inhabitants of Gaza.So in the case of Israel, Amnesty (and others) consider a closure of a territory over most of its borders to be "occupation."
Yet after the US and UK invaded Iraq, Amnesty in 2003 published a guide about the definition and legalities of a belligerent occupation. See how that definition, based on the Hague conventions, apply to Gaza:
The definition of belligerent occupation is given in Article 42 of the Hague Regulations:In the first quote from Amnesty above, it is clear that they recognize that Hamas has taken over Gaza. By their own definitions, which are very clear in the case of Iraq, "occupation" occurs only is when the outside army has physical control of the territory on the ground, and when it can override the decisions taken by the local authorities.
"Territory is considered occupied when it is actually placed under the authority of the hostile army. The occupation extends only to the territory where such authority has been established and can be exercised."
The sole criterion for deciding the applicability of the law on belligerent occupation is drawn from facts: the de facto effective control of territory by foreign armed forces coupled with the possibility to enforce their decisions, and the de facto absence of a national governmental authority in effective control. If these conditions are met for a given area, the law on belligerent occupation applies. Even though the objective of the military campaign may not be to control territory, the sole presence of such forces in a controlling position renders applicable the law protecting the inhabitants. The occupying power cannot avoid its responsibilities as long as a national government is not in a position to carry out its normal tasks.
The international legal regime on belligerent occupation takes effect as soon as the armed forces of a foreign power have secured effective control over a territory that is not its own. It ends when the occupying forces have relinquished their control over that territory.
The question may arise whether the law on occupation still applies if new civilian authorities set up by the occupying power from among nationals of the occupied territories are running the occupied territory’s daily affairs. The answer is affirmative, as long as the occupying forces are still present in that territory and exercise final control over the acts of the local authorities.
Any way you look at it, Gaza is not under the "authority" of the IDF and such authority over Gaza cannot be exercised by the IDF without a return of a physical presence. If Gaza is considered occupied because Israel has the ability to control its own border with a hostile territory, then Israel must have been "occupied" by its neighbors before 1977 because they also controlled their own borders and stopped any trade with Israel. The idea is absurd, and has no basis in international law.
In fact, Amnesty goes on in its 2003 guide to demand specific responsibilities of the US and UK in Iraq, responsibilities that it would be loathe to request Israel perform in Gaza. These include maintaining law and order, protecting civilians from being tortured or coerced, and other provisions that necessitate an actual physical presence in the territory. (Goldstone's legal arguments were even more bizarre.)
Amnesty's definition of occupation given in 2003 is quite accurate and fully in agreement with the Hague definitions. When it changes that definition concerning Israel in Gaza, it is engaging in blatant hypocrisy.