This is a great question. John is using a custom built gravity fall gate and so is the NPWDRL. I know that BSB was making gates for a while but has since moved on to a new hobby. I would take a look at Microwizard.com.
Reading the notes on that, the first thing that stands out is that you have to set the springs to the right tension for four, 5 oz cars and then tweak the adjustment if you run fewer, lighter, or heavier cars? So if your event includes Unlimiteds over 5 oz or if you want to test just one car you have to tweak it. I'd be afraid that it would affect the consistency of the track, but you could probably attach multiple anchors for the springs to the track and mark what each is for?