In a boost converter, should ground plane be placed underneath the gate trace?
In many high-power AC/DC designs that I've worked on, we generally divide the ground planes up so that the power return is kept clear of the control return. The two planes are connected together at a single point. Where possible we try to shield as much control as possible with the control return plane, even gate drive signals.

Bear in mind that a good MOSFET driver sources and sinks a lot of current, so as long as your gate trace isn't too lengthy it's quite difficult to induce enough signal on the connection to interfere with the MOSFET drive.