If I understand correctly, G at 440 hz sounds like Gb at 432 hz. It's 'off' by a semi-tone. If I'm not wrong on that, I could be, then could you play one instrument tuned at 440Hz (G note) and another instrument tuned at 432Hz (Gb note)? Would they sound in harmony or the same.

If not... if I had a tongue drum tuned at 432hz, is it impossible to play 'any' instrument (e.g., electric guitar) at 440 hz? Would they sound incompatible, no matter what notes are played on either?