The main landing gear door supports the weight of the retracted main gear when the landing gear system is depressurised.
More precisely, there is a gear support attached to each main gear door. When the door closes the gear support folds under and locks the main gear. The door is held closed by the door uplock hook. When the landing gear system is depressurised the weight of the gear rests upon the gear support.
During alternate extension, the uplock hook is released mechanically and the weight of the gear pushes the door open.
It is a little more sophisticated than the tyres resting on doors 
WRT to the tailskid and what holds it up. C HYD pressure does.
It has no uplocks, it is not pressurised in flight, what holds it up?
Incorrect, Unlike the gear, it IS pressurised in flight. The retract pressure is maintained by the centre system with the tail skid retracted as the control valve is in the same position with the landing gear lever UP or OFF.