It's a Z80-like core with some modifications; not all the Z80 instructions and registers are present and some instructions are completely different. Most instructions are the same as the 8080, but all 3 CPUs are not completely binary-compatible. It's neither truly an 8080 with added instructions, nor a Z80 with omitted ones.

For comparison: http://pastraiser.com/cpu/gameboy/gameboy_opcodes.html http://pastraiser.com/cpu/i8080/i8080_opcodes.html