If the landing gear is stuck in an airliner, can the crew go down and manually open it?
On older (WWII-era) large planes, the crew had access to a manual crank to lower the gear if the automatic extension/retraction system (usually a linear actuator powered electrically) failed. I don't know of any that had a gravity-based manual system; I'm sure a few bomber crews would have appreciated something like that after having to crank the handle to get a disabled main gear locked. Here are the manual crank ports for the B-17, on the forward bomb bay bulkhead with red arrows indicating the direction to crank:



The B-17 had a retractable tail gear as well, so there was a third manual crank in the tunnel to the rear gunner's position (but if there were multiple gear failures, the main gear would be the priority).

On more modern airliners with hydraulic extension/retraction, manually cranking the gear into place isn't as much of an option (especially since any access to a crank linkage would be in either the passenger cabin or the cargo hold). For these aircraft, the gear are designed with a "manual extension" or "gravity release". This basically disconnects the gear strut from its uplocks and the hydraulic cylinder, allowing gravity to pull the gear down and then momentum (or a spring mechanism) will engage the downlocks. Here is a diagram of the 737's manual gear panel on the right-hand wall of the flight deck:



The 747 has a manual nose gear extension system in the avionics compartment below the first-class cabin. The 747's main gear are much more complex, but it is still possible to let gravity pull them into place, by using a set of "alternate gear extension" switches on the overhead panel to release each gear from the hydraulics and uplocks:



It doesn't always work; a Virgin Atlantic 747 pilot was forced to make an emergency landing with the outboard right main gear stuck in a half-extended position: