Electrical (B134 [-20]) is an exotic physical Disadvantage where the body contains unshielded electronics, or relies on electrical power for its vital energy.
It can be applied as a Limitation to a Cybernetics implant. (B46)
As a Temporary Disadvantage as explained in https://web.archive.org/web/20180123014547/http://forums.sjgames.com/showthread.php?t=66065 it only applies to shutting down the disadvantage.
Power-Ups[]
GURPS Power-Ups 8 p 18 has the "Shutdown" variant explaining this.
Resistant to Electricity[]
GURPS Template Toolkit 2: Races talks about countermeasures on page 44.
It mentions that Immunity to "electromagnetic pulses, microwaves, and power surges" only costs 15 points.
It has "no effect on attacks that drain electrical energy" which is why it's still -5.
what that means for attacks isn't really clear, since Fatigue Attack and Leech can only target FP not ER