Gherkin running KMK on a PCA10059
It is possible to replace the Pro Micro on a PB-Gherkin with a PCA10059. The PCA10059 is running KMK firmware.
The PB-Gherkin PCB is flipped so that the "Pro Micro" would be mounted face up.
The PCA10059 is aligned with the bottom of the Pro Micro footprint, the two ground pins are skipped. The 6 column pins and 5 row pins match up with GPIO pins on the PCA10059.
There are pins that should not be soldered to the PCB (red X). They could cause shorts on those GPIO pins.
The green arrow points to the square pad, pin 1 on the Pro Micro footprint.
I had to use 14mm spacers to accommodate the thick USB extension plug. It is a tight fit, 15mm would be better.
KMK config
The KMK CircuitPython config file is here. It uses the unmodified KMK release files and only needs the Adafruit BLE library. I duplicated the original TMK Gherkin keymap as much as possible.
The PB-Gherkin PCB flipped the way it is (opposite of a normal Gherkin) rearranges the matrix. KMK has a method for rearrange the logical location of the switches with the physical positions.
It is also possible to enable BLE. The PCA10059 lacks any battery circuitry so this is only really useful for experimentation.