After getting the hardware I installed the software. Using the mentioned
- Raspbian Stretch with desktop Kernel 4.9
- gimx_6.11-1_armhf_stretch
- EMUPS4PAIRING (atmega32u4.hex)

I had some trouble getting the leonardo board bruned, but the "Arduino Builder" and "Arduino" 1.8.5 software helped me fixing it.

The trouble was getting the GIMX runnig as controller wit hthe PS4. I followed the Quick Start Tutorial for Ubuntu or Raspbian + bluetooth but it didn´t get the mouse and keyboard running/connected.

In the end I found out the following:
- the onboard bluetooth chip of the RPI doesn´t work, even it should be a CSR8510 (from Broadcom)
- In step 5 i was asked to "As requested, connect a Dualshock 4 with a micro USB cable, and click on "Yes"."
And attention (maybe it is obvious, but I struggled at this)
- this Dualshock PS4 should not be connected to the PS4 System! If it is tell the PS4 to "forget" it.
- And as it seems to be impossible to have two active controllers for one account, craete a new account for the GIMX-controller.

After doing this everthing went fluent. (but before ...)

Now I am still strugeling with the calibration (for Planetside 2), but that is another story ...