I totally get it now. And I just noticed that, actually, when I moved the x and y axis in the direction where the gizmos' arrow is pointing, the translation value will be positive. Only the z axis' translation will respond to the gizmos' pointing direction in a negative value, which is not consistent with other axes at all.
And with the understanding that when I open a 3D scene in Godot, I was always working behind the model, not in its front, I think I clearly see how things work in Godot now.
However, I personally think that the current implementation is very confusing. The main issue is that the gizmos' arrow is pointing backward only for the z axis. It's not just counterintuitive and inconsistent with other axes but it's also contrary to everything we learned in life, for instance, I would normally move forward to a direction where the arrow is pointing at. But in Godot, it's the opposite (and only for the z axis too).