That Harrowing took place before Burning Tides.
The following is how I understand that it went down: (not a Narrative team Rioter).
Miss Fortune was a well known bounty hunter with ships and crews to her name (that she had hired, welcomed, or tricked into working with her).
Gangplank knew of Miss Fortune and her bounty hunting ways - but Gangplank thought that the bounties on his head were hilarious! HE was the one who could ensure that his bounties were pride of place in the middle of the bounty board - DARING someone to come at him. In his mind, no-one would be stupid enough - and so it added to his legend.
When the Harrowing came, every ship and able seafarer was needed. Every weapon and warm body. Miss Fortune had weapons, ships, and manpower. Gangplank had weapons, ships, manpower, and influence. Of course, Gangplank appearing to need help to fight off the Harrowing would be a sign of weakness, so I like to imagine that he made a great show of 'inviting' people like Miss Fortune and her forces to come along and watch his victory.
Miss Fortune, burning with secret hate for Gangplank, would know how dangerous the Harrowing was, and that it would need every able-bodied soul to fight off.
Because of the terrible danger, Gangplank would have 'generously' invited Miss Fortune to attend. Miss Fortune would have put on her best fake smile, and accepted the invitation.
Because if they didn't - the town they both call home would have been full of nothing but wailing spirits within the week.