I'm going to go out on a limb and guess that this flashback means Gunko is going to be freed. But I really doubt that Loki would free Gunko even if Brook asked.
Loki doesn't even know who Brook is, he'd be like, why is this fucking skeleton talking, the hell? Absolutely no reason for Loki to free a celestial dragon who has attacked both him and the country.
So I think in terms of people that can melt Gunko, we're really looking at 3 dudes.
Luffy, Brook, and Sanji.
Luffy would be gear five bullshit. Sanji would be Flames of Love bullshit.
So I really hope that Brook is the one that melts this ice. Give him a legitimate power up.
I think they'll be able to rescue gunko, too.
I think you need to destroy the Seal/Mark of the Abyss, in order to free a God's Knight.
Luffy pulls the Seal off of Gunko, and Loki destroys it. If Loki was able to break through Harald's Seal, he should easily be able to break through Gunko's.
Luffy needs to peel the Seal/Mark off, though, the way his features flew off during egghead when Warcury roared.
Luff pulls the Seal/Mark off.
Loki destroys it.
Shuri is free, and can put on some pants.