Gun equalising

That's a very broad question. What exactly do you want to know?

As for the basics, it's the same as for a pneumatic gun. If one weld tip hits the work surface first, it stops moving and the other tip is pulled into contact, while applying minimal force to the work piece. Without equalization, the weld gun might apply unbalanced force and deform the metal.

Exactly how this is done depends on the gun design and manufacturer. Some have pneumatic counterbalances, some have springs, some actually tie back into robot motion and dynamically drive the robot to relieve force.

You're going to have to define your question more precisely. As I said, even with Servo guns, there are many different options, including springs, pneumatics, etc.