Let's make sure I am on the same page here. You are removing the solenoid; hence the need for the screw plug in the carburetor fuel bowl; right?
When you are removing the solenoid there is no need for the electrical circuit any more. It is only there to activate the solenoid therefore no longer needed.
The Grey wire is the +12V dc and simply deadheaded with tape or with whatever prevents it from grounding out. Here I would use head shrink tubing and crimp seal the end with a pair pliers after heating.
