If you use a toggle binding action, you need to move the gear up then down again. This is to simulate a “toggle” action where you are pressing a button and releasing it. Since the landing gear up is the only switch with an active signal, you have to trick the sim to “press and depress” by moving the landing gear up then down in a single move.
Same thing when you want to deploy the landing gear down, you do it by moving the TCA landing gear lever up then down again.
It’s a tricky stuff, The boeing has 3 states in the landing gear. UP - OFF - and DOWN respectively. So when you pull the landing gear up, once they’re stowed you have to move it to the Off position otherwise you’ll be at risk of breaking the hydraulic lines for having to keep pushing it up instead of releasing the pressure and locking it in like in the OFF position. (My real world 737 Pilot friend was shouting at me while watching livestream flying the PMDG 737 telling me to put the gear lever to the off position quickly after telling me that reason).
Obviously, we only have landing gear up and down position in the TCA Quadrant, so if we bind it to either Landing Gear Up with Button 21 on press and Landing Gear Down with button 21 on release. What happens is that while it will work stowing the gear, you can’t move it down to the OFF position because the TCA Quadrant is actively sending the signal to the sim that the landing gear is in the UP position, which overrides any action that you do with your mouse. But if you release it to the Down position, the aircraft will deploy the landing gear, which we also don’t want.
That’s the predicament here, the PMDG has 3 states of landing gear, while we only have 1 state of either ON/OFF in the TCA quadrant. The only way that we can get full functionality of the landing gear is to change it to Toggle Landing Gear. But make the movement of pushing the gear up then down, to stow the landing gear. Then you can manually use the mouse to move the gear lever to the OFF position, then as you approach your destination, do the same movement of gear up then down on the quadrant to deploy the landing gear again. As long as the default placement of the quadrant gear lever is in the down position, it will not send any signal to the sim.
I find that using Toggle Landing Gear to Button 21 On Release is also a good idea. This is to prevent the quadrant from sending an overlapping signal cycle or landing gear toggle, which will make the landing gear inside the cockpit to move rapidly between up and down when you hold the lever too long in the up position. By changing it to Button 21 On Release, the sim won’t activate the landing gear toggle command until you move the gear lever to the down position. So you won’t have that rapidly changing gears inside the cokpit.