Sorry your post has gone unanswered for so long- I was hoping one of the Picade gurus on this forum would pick it up, since I don’t get much time to play with a Picade/RetroPie so my expertise is very limited!
You could try whipping up a quick python script to switch your Picade PCB into Gamepad mode.
Try running sudo nano setuppicade.py and filling the file with this content:
import serial
picade = serial.Serial('/dev/ttyACM0',9600,timeout=1.0)
picade.write("g")
print(picade.readline().strip())
picade.write("s")
print(picade.readline().strip())
Save by pressing Ctrl+X, then Y, and enter to confirm.
And then run with sudo python setuppicade.py