Update:
I created a demo-project here with the board-definition for the Arduino GIGA. You can find it here.
Upload is working fine here, only drawback I noticed so far is, that it’s required to enter bootloader mode manually by double-pressing the the RST-Button.
I hope this helps someone.
Sadly I have no experience with the flash_layout and how to configure platformio to be able to flash the second core, too. Thus I will not create a pull-request, but hopefully my demo-project helps someone to start with creating a proper board-definition.