I’m having troubles with chaos vehicle myself, the behavior with the clutch/gear seems completely incorrect. 
I think your problem is that when you are in gear and press the clutch, you are setting the gear to 0, and upon release of clutch your setting it again back to 0 as that is now also the current gear since you set it as soon as the clutch was pressed. You are doing this twice in as well, so you can probably get rid of that extra node.
Instead, upon clutch release you should be setting it to the latest selected gear. That way releasing the clutch will go back to the correct gear.
edit: I quickly tried emulating what I think is your desired behavior. Could you try?
