Galvanic isolation of phone chargers
How come the (AC) voltage between earth and GND is a means to qualify the galvanic isolation of a phone charger? Shouldn't it be between GND and neutral? The earth is not connected in the circuit.

On the mains side, neutral and earth will be connected at some point (and maybe several points).  So measuring between power supply GND and earth is much the same as measuring between power supply GND and neutral (give or take a few volts).

When the galvanic isolation is not so great, is it GND or earth that oscillates? If this is GND, does it mean that VCC and GND are oscillating but the difference is always 5 V? If this is earth, what does it have to do with the charger?

By general consensus, the Earth is at 0V.  You can only measure voltage between two points, and it's helpful to have something that you can declare to be 0V as a reference for everything else.  So if the GND terminal of the power supply shows an AC voltage, then that's relative to the Earth's 0V.
You're seeing it because most switch mode power supplies have a class Y capacitor between the AC input and the DC output.  You're seeing a tiny current leaking through that.  The capacitor reduces electrical noise from the power supply.
Because your plug is reversible, you may see different results if you turn it around and plug it in the opposite way up.