The right to return is anchored in four separate bodies of international law: nationality law, as applied upon state succession; humanitarian law; human rights law; and refugee law. The following section presents nationality regulations in successor states. States have domestic discretion in regulating their nationality status, but this is limited by several binding obligations under international law.42
There are three main rights-obligations with regard to nationality law that some may urge obtaining a customary status, thus binding upon all states:
First, the right to obtain the nationality of the successor state: according to international law on the succession of states,43 the habitual inhabitants, regardless of their physical presence, of the geographical territory coming under new sovereignty are offered nationality by the new state.44
Second, the right of return is an obligation owed by a state to all other states: states are required to readmit their own nationals as refusal to do so would imposes an obligation to receive or host, the rejected individual on another state (the rule of readmission).
Third, the prohibition against (mass) denationalisation prohibits a state from using revocation of nationality status (i.e. denationalisation) as a way to avoid its obligation to admit its own nationals (Boling 2001).
http://cadmus.eui.eu/bitstream/handle/1814/8162/CARIM RR-2007-07.pdf?sequence=1
This would mean, for example, that 80% of the people in Gaza are Israeli citizens.