indexes: [
  {
    using: 'GIN',
    fields: ['field']
  }
]

// CREATE INDEX field ON $modelTable USING gin (field);

indexes: [
  {
    using: 'GIN',
    operator: 'jsonb_path_ops',
    fields: ['field']
  }
]

// CREATE INDEX field ON $modelTable USING gin (field jsonb_path_ops);

{
  field: {
    type: Sequelize.JSONB,
    index: true
  }
}

Should be converted to:

indexes: [
  {
    using: 'GIN',
    fields: ['field']
  }
]