Colon shouting for his father triggered a relapse in Imu's mind control of Gunko and she remembered her childhood with Brook as her apparent father. It explains her love for music especially his music too, as she was listening to his New World song earlier too.
She calls him by his name rather than as father, so maybe he's her foster father and it's another Kinemon and Kuma type situation where they adopted Momo and Bonney respectively ofc.
Imu sense Gunko was snapping out of their control over her, so they took full posession over Gunko.
Someone on Piratefolk was pointing out that Imu might have her mind under his control and thus brainwashed her and made her forget Brook BUT in her heart she still remembers him and that's what will triumph over Imu's mind control, supreme manipulation and order/power etc, literal power of love overcoming all:
https://www.reddit.com/r/OnePieceSpoilers/comments/1kse9xl
Also Gunko used a rapier made from her arrows to try and kill Colon, the same type of weapon Brook uses. She was about to kill another man's child because of Imu. So it relates that way as well.
HMMMMM:
Welp:
https://www.reddit.com/r/OnePieceSpoilers/comments/1kse9xl/_/mtm08ye
This storyline was really fucking dark and sad too^.
This is gonna be extremely relevant again now:
Subconsciously another reason for Chopper to perform a near suicidal sacrifice to try and save Scopper there too^. Bropper will save Scopper! It's like seeing another Hiriluk for him in a way, with Colon as a version of Chopper himself, as Hiriluk was Chopper's fathe figure too and sacrificed himself for Chopper!
Fuck man, sometimes I really fucking love this series so much. This is one of those times. For all the flaws of post TS has, theres been a ton of moments where Oda still delivers like he did pre TS and maybe better in some ways too.
OH SHIT:

