Gavigan declined to elaborate on how the debt had arisen, only saying it was a "long story".
Seaton Park director Scott Eagleton said the sale was "terribly disappointing".
He said had been dealing with Gavigan for many years but considered the forced sale his only option left.
"I haven't had to go this far before. I've had to sell his horses, but never like this before: It was always with his blessing," he said.
"He's effectively abandoned these horses, and I've had to get them registered. I'll be pleased if they can find a nice new home and I can stop worrying about them."
Gavigan has been championing a class action against the directors of Feltex, and IPO advisers Credit Suisse, for the better part of two decades.
The carpet-marker had floated in 2004, before infamously collapsing several years later and costing investors $250m.
Gavigan has been a key player in marshalling 3600 former investors claiming the IPOs and its prospectus was deficient, and lining up offshore litigation funding, but progress through the courts in the 15 years since has been drawn-out and uneven.
The marathon reached a climax in the Supreme Court in 2021 when the claimants, despite winning an element of their claim, had their case thrown out over a failure to provide $1.65m in security for costs.
Feltex directors pushed for claimants - including Gavigan - to be liable for more than $2m in costs, with the effective sum reduced to $438,000 on appeal.
Gavigan said the flurry of counterclaims had drawn his attention off the racecourse.
"You can understand that's taken more of my attention than racing horses," he said.
Gavigan expressed confidence the Feltex class action, after a new litigation funding structure was put in place, could be reinstated.
"We've never been closer to getting this to a compensation hearing," he said.