I don't own a G650XC but I know, that a lot of people wanted some more pic's/info.
Well, I went to the IMS show in Seattle and TT had the tank on display.
According to TT, the tank will hold 16 liters; cost around $800.00 to $900.00 here in the US and will be available within the next two months.
What I noticed: It is of white color but you can look through it to check the fuel level.
It is equipped with two petcock's at the lowest possible point(check attached pic's) and will be conected to the main tank.
Hope that helps.