One lightning attack would take out GG... Another more powerful one might defeat Obi Wan if not then other force attacks in combination with a lightsaber would.
Wielded by a force user yes,but could GG? I don't know if its because of the lightsaber or because of the power of the person.
I think its the person because Mace had a very hard time blocking Sidious his lightning, but Dooku his much weaker lightning was easily blocked by Obi.
And even if it could be blocked then he can use another technique in the force to kill GG in one second.
Are you insane? GG has no chance in hell against A dark lord of the sith, he was pwned by dooku in a practice fight and you put him up against a dark lord? Obi wan wouldent change anything. Sidious would take care of him first.
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When the darkness comes, keep an eye on the light no matter how far away it seems.