That looks like a nice servo for a robot elbow or knee. I found a specification page for it but I could not see any specification for side-load or radial-load. I doubt that it's strong enough to hold the weight of the gate. Put some normal hinges on the gate and make the servo operate it through a pushrod.
You should not use two servos in parallel like this. They will fight each other. They will always be a little out of sync with each other. Power consumption will be 2 or 4 times higher than if they aren't fighting. Just select one servo that is big enough to move the gate on its own. Add a helper spring or counterweight if you have to.
So servo.detach() doesn't seem to do the same thing as turning off the power to the servo? Then it seems like either your grandmother has to push a little harder or you need to turn off the power to the servo. Pololu makes some nice solid-state switches.