I would say "We have never used these tools again since you left" or "we have never used these tools again after you left." The reason is that "since you gone" is missing an auxiliary verb. You can say "since you have gone," but not "since you gone."

I think it would have been better if it was like " We have never used these tools again since when you are gone."

We haven't used these tools since you have been gone. or We haven't used these tools since you left. or We haven't used these tools since you were gone. To make it sound more "strong".. We haven't TOUCHED these tools since you were gone.