Quoted from TGP:
The GCB-95 is not true bypass, and nor is it buffered. Its bypass system only switches the output signal, while the circuit input is permanently connected ("hardwired", as dunlop calls it) to the input jack.
The reason it passes signal when powerless is because there is no active circuits in the direct signal path to the output jack. But that doesn't mean it is true bypass - that would require the switch to also disconnect the circuit input, which is something you can't test for by pulling the power.