You'll need to use the Simulator property on the physics controller, which gives direct access to the Farseer Physics simulator class. (Unfortunately the PhysicsControllerMain doesn't have setter's wired up for every property)
First make sure you add references to the FarseerPhysics.dll and Spritehand.PhysicsBehaviors.dll.
Then import their namespaces:
Then, you could do something like so, which sets the Vertical gravity to 500:
PhysicsControllerMain _physicsController = LayoutRoot.GetValue(PhysicsControllerMain.PhysicsControllerProperty)
_physicsController.Simulator.Gravity = new