Do airplanes have sensors to detect landing gear fires?
There are fire detection heat sensing loops in the gear well.  If a gear was on fire it would trigger a "GEAR OVERHEAT" message and warning when the gear was up.  There are no retraction prevention systems that are able to detect fires at the gear.  Nationair was a unique series of clusterflarks leading to the accident, on top of the fact that the magnesium wheels started to burn.  Modern magnesium alloys don't do that.