Bumped.
At least someone is trying to keep this topic alive, -sadcookiemonstersnip-
Do you have any plans to submit this to RTB?
how do you open the GUI?
To open the GUI you must set a key for it by going into Options->Controls, then scroll down until you find the CrossHair Exchange section and set the key for opening the GUI, I recommend setting it to F6.
How do you open the GUI in-game?
It says in the OP.