How to open a gcd file?

It's a binary file containing compiled native code.  What do you expect to be able to find?

Hardware parameters Chip model etc

There are multiple GCDs involved including GPS, Sensor Hub, BLE/BT/ANT, etc and from CIQ you can'T access any of the chips directly..  Only by way of the the CIQ APIs.

That's actually not correct Jim. The gcd file is a container which contains binary files. There is an extraction tool here https://github.com/mbirth/gcd-parser

This is a community reverse engineering effort. Usage of the tool is of course at your own risk.

And likely a copyright violation.....

Not sure anyone could get anything useful out of this.