Changes
The GBZ80 also lacks the NMI pin (thus no IFF2 and no RETN), the IM instructions and the I register. It has a different interrupt system than the Z80. [https://gbdev.io/pandocs/Interrupts.html Source]
The Nintendo documentation does not mention M-cycles or T-states at all. They only mention CPU cycles, which are always equal to 4 T-states (like NOPs in the CPC world). Also, the GBZ80 has different timings than the Z80. For example, :* CALL nn takes 6 cycles on the GBZ80, but only 5 NOPs on the Z80.* ADD HL,ss takes 2 cycles on the GBZ80, but 3 NOPs on the Z80
Even flags can differ: the DAA instruction clears HF in the GBZ80, but not in the Z80.
