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.
Even flags can differ: the DAA instruction sets HF to 0 in the GBZ80 , but not in the Z80.
Fun fact: Way more GBZ80 cores were produced for Gameboy hardware (118 million Gameboys and 81 million GBA) than all the Z80 chips produced for home computers and game consoles. [https://www.chibiakumas.com/z80/Gameboy.php Learn GBZ80 Assembly Programming with ChibiAkumas]