Essentially, there’s no talk of pitch bending as clocking covers the range of this. There are two methods. The first is an Over/Underclock that changes the clock speed on the fly, essentially creating a pitch bending function. Although as far as we know, there are no adjustable pitch bend mods that specifically step to notes, chords or octaves effectively.
The second, the most common is a direct switch from the processor to a crystal oscillator, at half the original speed of the Gameboy processor. This allows an octave down however, crashing is common using this second method, so the device, while having a ‘switch’ is mostly best used switched off.