When hacking is used in context of Gameboy games then I'd normally first think of using external tools such as a gameshark to directly edit memory, rather than game glitches. I'm not sure most people would say missingno glitch is hacking but it is a memory exploit.

Obtainable via memory exploits, if you want to call that hacking.

Gaijin. I think this knowledge was much more common on the internet a long time ago tbh.