Verification error after upload on Gboard Pro

I am working with a GBoard Pro which is Arduino like.
It can be programmed from the Arduino IDE with an FTDI USB to serial converter and selecting "Arduino/Genuino Mega or Mega 2560" as the board.

Uploading seems to work but always there is a verification error. Inspite of this error the sketches are running. I tried different sketches which were running successfully on a Mega 2560 board and could not find errors.

Now when I write some new sketch specifically for the Gboard Pro and there are some problems, I doubt if it was a programming error or an error of upload, because the verification error always is there.

Has somebody experience with the Gboard Pro?
Is "Arduino/Genuino Mega or Mega 2560" the correct board selection for it?
How to get rid of the verification error?

tips are highly welcome :wink: