As the GB hardware is more-or-less Z80 based, is your application emulating all the GB or "just" some parts ?
Anyway I am impressed !
Tetris was one of the first GB game, if not the first.
It's not a very demanding game, that's why emulating it on a CPC is possible.
Gameboy processor is very close to Z80.
I "only" needed for tetris to add emulation support for 8 extra instructions not present in Z80.
It doesn't use "tricky" graphics rendering, the graphics emulation engine just fit this.
Also sound emulation is very basic.