Does the GXM 54 come with a mounting bracket? Is there an extended mounting bracket that's taller it keep it away from the Garmin radar?

Yes, the Garmin Gxm 54 comes with both a surface mount as well as a pole mount. Here is the complete packing list: GXM 54, Pole mount, Surface mount, Mounting hardware, Cable (6 meters), Power and audio module, and Documentation