Gimbal Attachment

Can the sensor be attached to a gimbal such that it is always pointed straight down?

Yes, you can attach the sensor to a downward-facing gimbal. However, this is not absolutely necessary, since the roll/pitch angles of the copter are already accounted for in the ArduCopter code (for example, see this code). The advantage of adding a gimbal is that the copter can roll/pitch aggressively without the risk of losing sight of the beacon.