A quick check on Google Earth reveals that there’s plenty of sparsely populated, or unpopulated land in the Gaza strip. The media only ever shows us pictures of Gaza city, so everyone is left with the impression that Gaza is covered with hospitals, schools, mosques, and residential buildings.
Hamas are all war criminals. Locating implements of war in residential areas is against the Geneva Conventions. There are plenty of alternative locations — but, they would not generate the dead “innocent civilians” that are so important for Hamas propaganda.
I doubt Hamas ever signed the Geneva Convention since they are not a country (Hell, Gaza isn’t a country) and if they did sign it, the purpose would have been to sucker it’s enemies.