Did she even use lightning in the book? I know Wookieepedia refers to her attack against Luke as lightning, but I don't recall The Courtship of Princess Leia ever actually mention her using Force Lightning, and the way it's described how she took out Luke, it sounded more like a Nightsister spell of some kind. Luke wasn't cooked, shocked, ect. He was given a brain aneurism. Sure doesn't seem "lightning-y" to me.
Anywho, for the actual topic on-hand. Sidious was afraid of Gethzerion escaping Dathomir. So afraid that he dedicated considerable resources to completely blockade the planet. Why blockade an entire planet when you could just send your enforcer, Vader, down there to kill the threat? Because that threat might be powerful enough to kill Vader. (And unlike in most of the cases where Palpatine tries to kill Vader, there wouldn't even be a chance of getting a better replacement out of Vader's loss.) The way Courtship of Princess Leia reads, it legitimately seems as though Gethzerion is a Sidious-level foe. I think in Round 1, she'd make it at least to Plagueis, possibly even beating him. And with how well we know Talzin was able to hold up against Sidious in Son of Dathomir, I think Gethzerion has a serious chance of threatening RotS Sidious. Don't think she takes RotJ Sidious, but it'd still be close.
(For the record, I'm comparing Gethzerion to Talzin here specifically because Sidious seemingly felt even more threatened by Gethzerion than he did by Talzin. He didn't try to blockade Dathomir when Talzin was in charge, and instead had Dooku send Grievous to try and take her down. When Gethzerion was in charge, he refused to let even a single ship land on the planet for fear that Gethzerion would escape and wreck havoc upon his rule of the galaxy.)
Off Dathomir, she's definitely going to be weaker. However, Talzin, while away from Dathomir, was able to put up a fight against Mace Windu for a short while, and the power of her lightning didn't seem all that diminished. Talzin was also able to perform some impressive feats of Nightsister sorcery while off Dathomir, being able to fade into and manifest from mist, summon a blade of energy capable of matching a lightsaber, ect. I'd say off Dathomir, Gethzerion might make it to Dooku before meeting her match, if the amount of power she loses is comparable to what happened to Talzin.
Honestly, I really want to see a match between Talzin and Gethzerion. Sure, Gethzerion doesn't have many feats, but they were both powerful and respected leaders of Nightsister clans, and they were both perceived as threats to Sidious himself.
Found the scene where Gethzerion attacks Luke. Doesn't appear to be Force lightning at all.
Luke explicitly refers to what Gethzerion did as a "spell," so it was neither TK nor Force Lightning. She later does the same trick against General Melvar and over a hundred stormtroopers. Han witnesses Gethzerion make 'a raking motion with her index finger' and suddenly Melvar clitches his temple before staggering forward and dying. Over a hundred stormtroopers around them all die in a similar staggering motion. So clearly, Gethzerion was using Nightsister magic to give people brain aneurysms, not throwing around Force lightning in the book. There is a different Nightsister who was described using lightning in The Courtship of Princess Leia, but it wasn't Gethzerion.
Also worth noting (irrelevant to this thread as it is) that the line where Luke refers to "if Vader had tried to kill [him]" comes after Luke was dealt a serious brain injury, and his mind is dying. So it's an inappropriate line to be used for scaling and saying whether or not RotJ Vader was greater than RotJ Luke, since Luke was an unreliable narrator at the time he thought the line (dying from a bleeding brain and all).
Yeah plenty of sources including Vader himself say Luke>=Vader by ROTJ. Cronal says he is stronger than him 6 months after ROTJ.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."