palgia,
It's not at all uncommon to see a Mach Number limit for Gear AND for Slats / Flaps. In fact, for 6 of the 7 heavy jet aircraft which I've operated, predominantly Boeing, all had a Mach limit for the gear (the odd one out was A300 and I cannot recall, it was too long ago).
In the case of the gear, there are 2 main concerns -
(1) The very unaerodynamic gear assembly has a considerably lower Mcrit than the rest of the aircraft. Drag increase beyond it's Mcrit would be rapid and severe, causing excessive pitch-down moments, and
(2) Shock wave formation on the gear door assemblies causes high torsional stresses (as opposed to dynamic pressure from the subsonic consideration). These torsional stresses oscillate due to the fairly low rigidity of the doors, causing flutter and inevitable separation from the aircraft.
Some manufacturers (like Douglas used to) published a Max Operating Mach No. for Slats / Flaps, others simplify the situation by imposing a cut-off Flight Level (Typically F/L 200) for Flap operation at which Pressure Height the Flap Limit CAS is below the limiting Mach No.
(Away for 6 days - look forward to further responses)