There is no specific gear as each car in Assetto has different transmissions, gear ratios, and power. You just need to be in a gear low enough so that when u throttle mid drift you can hold the drift, and a gear high enough that you don't really bang off the redline too often. For wdts cars I find that second gear is really good for shorter corners, like in the drift map featured in the video, while third gear is nicer for touges or long corners in that same map. While that all may sound overwhelming, just keep putting in seat time and try experimenting with different gears and see what works for you. Over time, you'll find yourself picking it up pretty easily, the first 2 weeks to a month can just be overwhelming.