I actually took this tool from adafruit: https://github.com/adafruit/Adafruit-Retrogame/blob/master/README.md
With this you can easily remap your GPIOs to a virtual keyboard without resoldering it. I don't know any other way to do that. If I try to configure my controller without "retrogame" and with
controllers.gpio.enabled I am not able to use every button.
I think this is the reason why retrogame does not work:
Beside keyboard as a gamepad is not supported in recalbox, most emulators won't work
But in the meantime I figured out, that I had to to the follwing. Now the buttons start, x and y work, but not the whole set. I also opened an issue on github as that place may be a bit more specific for a possible solution.
Anyway. Here is how the GPIOs are connected to my buttons:
A > 21
B > 16
Start > 13
Select > 18
Right > 22
Down > 05
Up > 04
Left > 14
X > 20
Y > 26
Here is also an example config from Adafruit Tools retrogame: https://raw.githubusercontent.com/adafruit/Adafruit-Retrogame/master/configs/retrogame.cfg.zero
If you could provide me any other "HowTo" that enables me to use all of my buttons in recalbox without rewiring it, I would really appreciate it.
By the way: In retropie everything works like a charm. But I don't want to use it! :)