We haven’t used the Ph Sensor before, it would be great if you could provide us the link to the sensor which you are using, so that we can double check the specs and help you better.
There are three i2C ports on the GrovePI that you can use and you might also have to strip the cables and make a customised cable specific to the sensor used.
I think you will have to customise the cable for connecting this Ph sensor to the I2C port.The I2C port of the GrovePi has 4 pins and you will have to map it to the pins in your sensor. You can refer to the I2C pinout in the image below.There are three I2C ports on GrovePi and you can use any one of them.
Yes you can use the Gpio Pinout or the I2C port of GrovePi for I2C communication. Raspberry Pi works on 3.3V logic and hence you must use Pin 1 instead of Pin 4. Also use Pins 3 and 5 for I2C communication and Pin 6 for ground.
If you wish to use the I2C port of GrovePi then you will have to buy the converter that you have posted.