A retract gear that locks is one thing.
A retract servo is another thing.

Most retract gears are designed so that when the gear is fully opened or fully closed, the strut is held and supported by the structure of the mechanism. No effort is needed on the pushrods to resist the force of landing nor to hold the strut/wheel up in the wheelwell. When the gear is up, there is no load on the pushrods. When the gear is down, there is no load on the pushrods.

Retract servo's are designed so that when the servo arm is at either end of it's travel, the servo motor is "turned off." That way, the servo is not constantly working to hold the gear down or up. It only works to move it. That saves constant drain on the battery. When the servo is all the way "up" it will not draw from the battery. When it is all the way "down" it won't draw.

The problem you have with a standard servo, even with good retract mechanism design, is that unless the linkage is adjusted perfectly a standard servo will feel a load and fight it. If the linkage is too long and pushes or pulls the retract unit past either "at rest" location, the standard servo will work and work and work and drain the battery. If the linkage is too short etc etc

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