I unfortunately didnt find this information from Oxinarf. Now that I've found it, I understand and I've fixed my issues (I2C related on Pixhawk 2.1 only).
If you want this I2C device to work, you need to hack into the I2C located on the GPS1 port - until they implement support for the other I2C busses.
I have a HERE gps. In my case I spliced the following wires, spliced a 4pin I2C cable in (do it nice - use solder and shrink tube and make sure your connections are done well, or find someone who can) Here connector is with the pins up - purple on right - I2C cable is pins up - red on right:
HERE GPS - Pin 1 Purple 5V = I2C cable Pin 1 (Red)
HERE GPS - Pin 4 Yellow I2C SCL = I2C cable Pin 3
HERE GPS - Pin 5 Orange I2C SDA = I2C cable Pin 2
HERE GPS - PIN 8 Black Gnd = I2C Cable Pin 4
Plug this 4 pin cable into your old I2C splitter, or if you used a new JST cable (like what comes with PH2.1) and want to use your new I2C splitter, it's the same pinout - pins up - VCC on right.
Don't feel real great about hacking into the new GPS cable, but I have several I2C devices and would like to both use them and test new firmware with my new Pixhawk 2.1. And now I can.
Hope it helps.