A wave trap is a band-stop filter. They are tuned to block a carrier frequency that is injected into the transmission line. Line tuner, on the other hand, is a band-pass filter. Both work hand-in-hand implementing power line carrier pilot protection.
Surge arresters are primarily used to discharge lightning strikes and switching surges to ground – protecting equipment in the process. Note that arresters are not applied to relieve the system from temporary over-voltages.
Air has a dielectric strength of 33kV/cm. SF6 gas, on the other hand, has a dielectric strength of ~83kV/cm. Naturally, when the live parts are immersed in a pressurized gas chamber they can be placed a lot closer.